user-list Databases Value is JSON vs a String

changes the 'databases' value returned in user-list from json to
a string of comma separated values

$ trove user-list <instance_uuid>

before:
+------+------+-----------------------------------------------+
| name | host |                   databases                   |
+------+------+-----------------------------------------------+
| bob  |  %   |             [{u'name': u'monkey'}]            |
| tom  |  %   | [{u'name': u'freshdb'}, {u'name': u'monkey'}] |
+------+------+-----------------------------------------------+

after:
+------+------+-----------------+
| name | host |    databases    |
+------+------+-----------------+
| bob  |  %   |      monkey     |
| tom  |  %   | freshdb, monkey |
+------+------+-----------------+

Change-Id: I8e18e05b722be8a65a15f9ed15264d48d50ecc4e
Closes-Bug: #1238216
Merge "user-list Databases Value is JSON vs a String"