Updating python-glanceclient to 0.15.0 release

Primarily, this addresses some forward compatibility
API changes happening in the Glance service. Without this
release, devstack deployments will have errors using the
v2 API due to patches added in Kilo.

In addition it contains patches that both Nova and Horizon
required.

Release notes: https://review.openstack.org/#/c/140079/
Other Release notes since the requirements were last
updated:
https://review.openstack.org/#/c/135267/

Change-Id: I5e3e1cd630505f735fa3c8a25a872769db48ff3b