Fix nova_cell_v2_discover_host.py with python3

When this script runs via python3 it fails with:

  "stdout: ",
  "stderr: + command -v python3",
  "+ python3 /docker-config-scripts/nova_cell_v2_discover_host.py",
  "Traceback (most recent call last):",
  " File \"/docker-config-scripts/nova_cell_v2_discover_host.py\", line 75, in <module>",
  " TypeError: a bytes-like object is required, not 'str'"

This is because in python3 subprocess.check_output() will return bytes
and trying to split it using a string '\n' will break with the error
above. Let's just use the universal_newlines=True parameter which we
have been using everywhere in tripleo so far.
Also skip any empty lines that might show up in the output which would
give the error:

  ValueError: not enough values to unpack (expected 2, got 0)

Tested with a python3 deployment and got a successful deployment (rhel8
os + f28 based-containers).

Change-Id: Ic7904c4f3027cc5e7e05d52757e36dbc05f3d487
Co-Authored-By: Damien Ciabrini <dciabrin@redhat.com>
Closes-Bug: #1813014
(cherry picked from commit 87a869a408)

docker_config_scripts/nova_cell_v2_discover_host.py

@@ -48,7 +48,7 @@ try:
         '/etc/nova/nova.conf',
         'DEFAULT',
         'host'
-    ]).rstrip()
+    ], universal_newlines=True).rstrip()
 except subprocess.CalledProcessError:
     # If host isn't set nova defaults to this
     my_host = socket.gethostname()
@@ -58,7 +58,7 @@ except subprocess.CalledProcessError:
 retries = 10
 for i in range(retries):
     try:
-        service_list = subprocess.check_output([
+        service_output = subprocess.check_output([
             'openstack',
             '-q',
             '--os-interface',
@@ -72,8 +72,12 @@ for i in range(retries):
             'Zone',
             '-f',
             'value'
-        ]).split('\n')
+        ], universal_newlines=True)
+        service_list = service_output.split('\n')
         for entry in service_list:
+            # skip any empty lines
+            if not entry:
+                continue
             host, zone = entry.split()
             if host == my_host and zone != 'internal':
                 print('(cellv2) Service registered, running discovery')