Remove deprecated option volume_feature_enabled.api_v1

volume_feature_enabled.api_v1 config option to enable the
Cinder v1 API was deprecated in Juno
- https://review.openstack.org/#/c/446238/

Cinder v1 API tests were also got removed from Tempest in Juno
- https://review.openstack.org/#/c/446233/

This commit remove the deprecated api_v1 config option and also
remove the cinder service clients alias from clients.py

Note: clients.py cinder v1 service clients alias are being used by
many plugins (depends-on patches). These are being used mainly in
scenario manger copy of plugins.

Depends-On: https://review.openstack.org/#/c/573168/
Depends-On: https://review.openstack.org/#/c/573167/
Depends-On: https://review.openstack.org/#/c/573164/
Depends-On: https://review.openstack.org/#/c/573174/
Depends-On: https://review.openstack.org/#/c/573169/
Depends-On: https://review.openstack.org/#/c/573163/
Depends-On: https://review.openstack.org/#/c/573171/
Depends-On: https://review.openstack.org/#/c/573175/
Depends-On: https://review.openstack.org/#/c/573172/
Next we will deprecated the cinder v1 service clients present in
https://github.com/openstack/tempest/tree/db876f990d1a2b79ca0149483bba4decd0e299a2/tempest/lib/services/volume/v1

Only networking-fortinet left to merge the changes which seems inactive
now a days- https://review.openstack.org/#/c/573170/

As reminder sent in ML, we are good to merge this patch
- http://lists.openstack.org/pipermail/openstack-dev/2018-August/133189.html

Change-Id: Ief961758ee413d69d89d45a1b36f1d126c6101cf
4 files changed