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
Fix undefined variable in cache

A function to verify metadata in the cache middleware was falling over
for GETs on images with a missing size.

Fixes bug 1183049

Change-Id: I2da75e5e5aca12d20a10cace2a3779e4041161b2
Sync oslo imports to the latest version.

Change-Id: I36193d00930e5c82c46841b64650863c811b95a4
Import strutils from oslo, and convert to it.

The only change of behaviour here (according to the former glance
unit tests) is that 'T' now means True not False. I suspect that's
ok.

Change-Id: Icf98f913a3f78a1fe2f38a82a25a071c2da03739
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"
Merge "Sync oslo imports to the latest version."
Merge "Import strutils from oslo, and convert to it."
Merge "Fix undefined variable in cache"