tornado: Remove explicit tornado_processes setting; compute it.

We can compute the intended number of processes from the sharding
configuration.  In doing so, also validate that all of the ports are
contiguous.

This removes a discrepancy between `scripts/lib/sharding.py` and other
parts of the codebase about if merely having a `[tornado_sharding]`
section is sufficient to enable sharding.  Having behaviour which
changes merely based on if an empty section exists is surprising.

This does require that a (presumably empty) `9800` configuration line
exist, but making that default explicit is useful.

After this commit, configuring sharding can be done by adding to
`zulip.conf`:

```
[tornado_sharding]
9800 =              # default
9801 = other_realm
```

Followed by running `./scripts/refresh-sharding-and-restart`.

scripts/nagios/check-rabbitmq-consumers

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 import argparse
-import configparser
 import os
 import subprocess
 import sys
@@ -11,6 +10,7 @@ from typing import Dict
 ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 sys.path.append(ZULIP_PATH)
 from scripts.lib.check_rabbitmq_queue import normal_queues
+from scripts.lib.zulip_tools import get_config_file, get_tornado_ports
 
 states = {
     0: "OK",
@@ -33,13 +33,8 @@ parser.add_argument('--min-threshold',
 
 options = parser.parse_args()
 
-config_file = configparser.RawConfigParser()
-config_file.read("/etc/zulip/zulip.conf")
-def get_config(section: str, key: str, default_value: str) -> str:
-    if config_file.has_option(section, key):
-        return config_file.get(section, key)
-    return default_value
-TORNADO_PROCESSES = int(get_config('application_server', 'tornado_processes', '1'))
+config_file = get_config_file()
+TORNADO_PROCESSES = len(get_tornado_ports(config_file))
 
 output = subprocess.check_output(['/usr/sbin/rabbitmqctl', 'list_consumers'],
                                  universal_newlines=True)