parser.Stack add stored_context

Adds a new stored_context method, which may allow for some cleanup
in service.py, and also enable switching from the request context
to the stored context when needed, e.g when the current context
auth_token is about to expire.

Change-Id: I7b797719036238c424905a5d2555dc00e40f4e89
Partial-Bug: #1306294
Restore resource metadata attribute

For backwards compatibility the Resource.metadata attribute
has been restored, however reading or assigning this attribute
will result in a deprecation warning.

Reading metadata will force a refresh from the database, which
maintains the previous behaviour rather than optimising for
performance.

Change-Id: Ifdcf3d98b99a09aac8755c84ef3f59d8602fc6a8
Closes-Bug: #1324704
Assume compute service_type for nova client

This change removes the ability to specify a non-compute service
type when requesting a nova client. The only non-compute service
type actually requested was the fallback path for cinder. However
nova volumes was completely removed from nova in grizzly, so we
do not need to support this any more.

Change-Id: I9e15f31979d44f018db69ebc8109f707b0f51671
Merge "Restore resource metadata attribute"
Merge "Assume compute service_type for nova client"
Merge "parser.Stack add stored_context"