Fix "create database" failed for postgress

This PR stops using the psycopg2’s connections context manager
because "CREATE DATABASE" command must be run outside any
transaction[1].
[1]: https://www.psycopg.org/docs/usage.html#transactions-control

Co-Authored-By: hungnt1 <sudo.nguyenhung@gmail.com>
Co-Authored-By: Hirotaka Wakabayashi <hiwkby@yahoo.com>

Story: 2010761
Task: 48059
Change-Id: I73ec6c659d8ad7216460055077737429c878df33

releasenotes/notes/fix-postgress-create-database-75fbe03e3b4e296d.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix guest-agent failed to start postgres container due to execution
+    of the "CREATE DATABASE" statement within the context manager of
+    psycopg library. See the following for details
+    `Stroy 2010761 <https://storyboard.openstack.org/#!/story/2010761>`__
+

trove/guestagent/datastore/postgres/service.py

@@ -770,12 +770,15 @@ class PostgresConnection(object):
     def _execute_stmt(self, statement, identifiers, data_values, fetch,
                       autocommit=False):
         cmd = self._bind(statement, identifiers)
-        with psycopg2.connect(self.connect_str) as connection:
+        connection = psycopg2.connect(self.connect_str)
         connection.autocommit = autocommit
+        try:
             with connection.cursor() as cursor:
                 cursor.execute(cmd, data_values)
                 if fetch:
                     return cursor.fetchall()
+        finally:
+            connection.close()
 
     def _bind(self, statement, identifiers):
         if identifiers: