Stop altering the glance API URL

Horizon should treat the glance API endpoint URL the way it treats
other API endpoint URLs.
Horizon should trust that the retrieved URL is correct, and let the
python-glanceclient use the URL.

Change-Id: Ie6230b1e4b90f686c8a7ab7368944b6c3412b284
Closes-Bug: 1188182
Merge "Stop altering the glance API URL"