gluster: Fix add host gluster uuid already exists in the db.

Check whether add host uuid already exists in the database
and throw an appropriate error message.

Change-Id: I1cca4c64cdbcb6a681089e1fbb2f0d707f768083
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=992899
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>