Improve clarity, encapsulation and performance of assignment type calculation

[Alexander Makarov]

There is an assignment type calculation function in:
https://github.com/openstack/keystone/blob/master/keystone/assignment/backends/sql.py#L121-L135,
which is better to be implemented as an AssignmentType classmethod.

[Alexander Makarov (amakarov)]

https://review.openstack.org/#/c/141352/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/141352
Committed: https://git.openstack.org/cgit/openstack/keystone/commit/?id=807cd726ddd15fc335438008acf9cec0a8e43398
Submitter: Jenkins
Branch: master

commit 807cd726ddd15fc335438008acf9cec0a8e43398
Author: Alexander Makarov <email address hidden>
Date: Fri Dec 12 15:35:09 2014 +0300

    Assignment sql backend create_grant refactoring

    Moved calculate_type function to be a class method of AssignmentType.

    Change-Id: Ib0ae2c04ab8b428a8f680520d53fbeb799fae6f0
    Closes-Bug: 1403929