blueprint devstack-support-for-keystone-mixbackend

Added KEYSTONE_ASSIGNMENT_BACKEND to support mixed backend

Modified code for KEYSTONE_IDENTITY_BACKEND according to comments.
Implemented the check for variables instead of case statements.
UPD: Removed arrays.
UPD2: fixed spacing issues
Change-Id: Ie92eed1fb5be5f875ef6633ede9c9e08daf6bf4f
Implements: devstack-support-for-keystone-mixbackend
Merge "blueprint devstack-support-for-keystone-mixbackend"