Run django.setup() in nagios plugins to avoid exceptions on 1.8

If running on Django 1.8, running these plugins would die with the below. A fix
for this is to run `django.setup()` before interacting with Django.

Refs:
    https://docs.djangoproject.com/en/1.8/ref/applications/#troubleshooting

```

Traceback (most recent call last):
  File "/usr/lib/nagios/plugins/check_send_receive_time", line 103, in <module>
    sender = get_user_profile_by_email(settings.NAGIOS_SEND_BOT)
  File "/home/zulip/deployments/current/zerver/lib/cache.py", line 113, in func_with_caching
    val = func(*args, **kwargs)
  File "/home/zulip/deployments/current/zerver/models.py", line 1073, in get_user_profile_by_email
    return UserProfile.objects.select_related().get(email__iexact=email.strip())
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 328, in get
    num = len(clone)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 144, in __len__
    self._fetch_all()
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 977, in _fetch_all
    self._result_cache = list(self.iterator())
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 238, in iterator
    results = compiler.execute_sql()
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 829, in execute_sql
    sql, params = self.as_sql()
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 378, in as_sql
    extra_select, order_by, group_by = self.pre_sql_setup()
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 48, in pre_sql_setup
    self.setup_query()
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 39, in setup_query
    self.select, self.klass_info, self.annotation_col_map = self.get_select()
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 206, in get_select
    related_klass_infos = self.get_related_selections(select)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 700, in get_related_selections
    [f.name], opts, root_alias)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1471, in setup_joins
    names, opts, allow_many, fail_on_missing=True)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1372, in names_to_path
    if field.is_relation and not field.related_model:
  File "/usr/lib/python2.7/dist-packages/django/utils/functional.py", line 60, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 110, in related_model
    apps.check_models_ready()
  File "/usr/lib/python2.7/dist-packages/django/apps/registry.py", line 131, in check_models_ready
    raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
```

puppet/zulip_internal/files/nagios_plugins/check_send_receive_time

@@ -16,6 +16,8 @@ import random
 import traceback
 import os
 
+import django
+
 def total_seconds(timedelta):
     return (timedelta.microseconds + (timedelta.seconds + timedelta.days * 24 * 3600) * 10**6) / 10.**6
 
@@ -64,6 +66,9 @@ import zulip
 
 sys.path.append('/home/zulip/deployments/current')
 os.environ['DJANGO_SETTINGS_MODULE'] = "zproject.settings"
+
+django.setup()
+
 from zerver.models import get_user_profile_by_email
 from django.conf import settings
 

puppet/zulip_internal/files/nagios_plugins/check_user_zephyr_mirror_liveness

@@ -11,7 +11,12 @@ import datetime
 import os
 import sys
 
+import django
+
 os.environ['DJANGO_SETTINGS_MODULE'] = "zproject.settings"
+
+django.setup()
+
 sys.path.append('/home/zulip/deployments/current')
 sys.path.append('/home/zulip/deployments/current/zerver')