Make passing user token to registry configurable

This patch adds CONF.use_user_token as a config option. When true, glance will
continue to use the user token when communicating with glance registry. When
false, glance will read admin credentials from the config and use those to
communicate with glance registry. Using admin credentials allow for
reauthentication when tokens expire and prevents requests from silently failing.

DocImpact
Fixes bug 1182536

Change-Id: Ia6b563677eb67d4069571c82b9dad3f025b6e9fb
Add documentation on reserving a Glance image

This lets us refer to the fact that some optional headers are in fact
required unless reserving an image.

Fixes bug 1124224

Change-Id: I5646d4dc130efecf991573b363eb1a665012b7df
Merge "Make passing user token to registry configurable"