MB-1143: don't attempt to use rackawareness API in mixed versions

When we're in mixed version cluster we don't have advertised server
groups urls. And we're detecting that as indication that rack awarness
ui features should not be used.

Change-Id: I4fb21114c672cbfe3d00bc904531edac1cd358e7
Reviewed-on: http://review.couchbase.org/30300
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>