fix network discovery meters

currently we let neutronclient authenticate against keystone. this
makes it impossible for us to figure out whether service is valid
until too late (when we call poll neutronclient).

this patch adds keystoneclient to check_keystone decorator so it
can verify endpoints when keystoneclient is not available.

Closes-Bug: #1361818
Change-Id: Ia374e9b422ed8fa088b28ecf200161248136b09e
Merge "fix network discovery meters"