Included 'allowed_direct_url_schemes' in driver options of
cinder configuration with a default value of 'cinder'
Also, added to unit test and functional test as part of change.
- modified cinder_purestorage.py
- modified tests_cinder_purestorage.py to include a functional test
- modified test_lib_charm_openstack_cinder_purestorage.py to
extend the unit tests
- rebased
Closes-Bug: #1957848
Change-Id: I26a033b03f6d03fa5e68d9e26aa741c6529ecac2
ee5c5d92d8