Raises BadRequest for grant_access on ignore_dbs

Reasons:
- Currently, it just logs an informative log in guestagent log, and then
  control shifts to next line, which provides the requested grant.
- This brings the need to raise the exception instead of informative
  logging.

Changes:
- Raises BadRequest if ignore_dbs are requested access instead of
  informative logging into guest-agent logs.

Change-Id: I9df08d04cd42916f3d1f91e05cd4799bb9041eae
Closes-Bug: #1257021

trove/guestagent/datastore/mysql/service.py

@@ -384,7 +384,7 @@ class MySqlAdmin(object):
                     try:
                         mydb.name = database
                     except ValueError:
-                        LOG.info(_(
+                        raise exception.BadRequest(_(
                             "Grant access to %s is not allowed") % database)
 
                     g = sql_query.Grant(permissions='ALL', database=mydb.name,