identity: configure admin_endpoint URL

puppet-keystone use admin_endpoint in keystone.conf for getting a token
with a provider.
If the flag is not set, it will use HTTP and we need to be able to
dynamically configure this flag to allow using HTTPS.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #519 from enovance/admin-endpoint

identity: configure admin_endpoint URL