UA server use schema for postgres table and remove cassandra requirement

[sjones4]

The UA server is updated to use the created schema for postgres. When using postgres we now no longer require cassandra is running.

[sjones4]

Daily build no. 6967

[sjones4]

Demo is that the table is created under the appscale_ua schema:

psql (10.10 (Ubuntu 10.10-0ubuntu0.18.04.1))
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, bits: 256, compression: off)
Type "help" for help.

appscale-db=> \dn
    List of schemas
    Name     |  Owner   
-------------+----------
 appscale_ua | appscale
 public      | postgres
(2 rows)

appscale-db=> \dt appscale_ua.*
             List of relations
   Schema    |   Name   | Type  |  Owner   
-------------+----------+-------+----------
 appscale_ua | ua_users | table | appscale
(1 row)

appscale-db=> \dt 
Did not find any relations.
appscale-db=>