Added flavor=keystone to to glance-api-paste.ini file

Added flavor=keystone parameter to conf file.

Change-Id: Icd44fc4df20f89491da0cb1d007ee8ef69b5b3b3
backport: havana
Closes-Bug: #1256379
(cherry picked from commit e5bbc25e95d199594d78de32356b52fc1d9ca302)
Merge "Added flavor=keystone to to glance-api-paste.ini file" into stable/havana