Only initialize the glance_http if service is enabled

This commit just adds a check to see if glance is set as enabled in
the config file before initializing the glance http client. Previously
regardless of whether glance was enabled or not glance_http would
auth against keystone.

Closes-Bug: #1214774
Change-Id: I07127c21d55d58fdb294468a0b74ce2bc54d8719
Merge "Only initialize the glance_http if service is enabled"