Remove 'openstack/common/context.py'

context.py is not used anywhere in Glance. Consequently, we can get
rid of it.
Also, context.py is the only file left using uuidutils.

Change-Id: I657e20ac39fbace60f230652912a4bf792836206
Closes-Bug: #1253497
Merge "Remove 'openstack/common/context.py'"