Return 400 if create volume snapshot force parameter is invalid
Return 400 if create volume snapshot force parameter is invalid

Fixes bug #1014689

* Add is_valid_boolstr function in utils.py
* Add force parameter check in SnapshotsController.create()
* Add unittest for invalid force parameter.

Change-Id: Ie6a07a2ac589da76f52a82b126a6d66e5987edc4

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects