parser.Stack add use_stored_context option

Add an option to switch to a context based on the user_creds data
at constructor time, which allows a cleaner interface for selecting
the stored context than creating it before creating the Stack object.

Partial-Bug: #1306294

Change-Id: I44df4e142dfbba1e70199b0d8f14c910f028a0f1
Merge "parser.Stack add use_stored_context option"