provision: Rewrite using subprocess module instead of sh.

The `with sh.sudo` pattern that we were using in python-sh was
deprecated, and emperically hangs on Ubuntu xenial.  Since in general
the use of python-sh/python-pbs caused trouble (requiring extra
dependencies, confusing syntax), this just removes it.

We replace it with a new zulip_tools.py library function that echoes
the command line and streams the output.

We do the same to install-phantomjs so we can remove that dependency.

zulip_tools.py

@@ -5,6 +5,7 @@ import errno
 import os
 import pwd
 import shutil
+import subprocess
 import sys
 import time
 
@@ -67,3 +68,12 @@ def get_deployment_lock(error_rerun_script):
 
 def release_deployment_lock():
     shutil.rmtree(LOCK_DIR)
+
+def run(args):
+    # Output what we're doing in the `set -x` style
+    print("+ %s" % (" ".join(args)))
+    process = subprocess.Popen(args)
+    rc = process.wait()
+    if rc:
+        raise subprocess.CalledProcessError(rc, args)
+    return 0