Add a new configuration option "backend_type" into tempest.conf

[Vincent Hou]

If we enable tempest service in ENABLED_SERVICES, for the [volume] section in tempest.conf, we will generate the following:
[volume]
backend2_name = lvmdriver-1
backend1_name = storwize1
In localrc, we actually need to set CINDER_ENABLED_BACKENDS=storwize:storwize1,lvm:lvmdriver-1 for multiple backends support.

However, only backend_name in tempest.conf does not directly indicate the back-end driver. We need to save the backend_type, which is in front of ":" to tell tempest what kind of back-end it is. Propose to add the config option backend_type in tempest.conf, so the config is like:
[volume]
backend2_name = lvmdriver-1
backend2_type = lvm
backend1_name = storwize1
backend1_type = storwize

Why is it useful?
I am implementing a spec: https://review.openstack.org/#/c/186327/, which proposes to add tempest tests for volume migration across different back-ends, for example, between LVM and storwize. If the devstack is configured with multiple back-ends, I need to what kinds of back-end maps what back-end name, what back-end type is available in the devstack env, etc.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Vincent Hou (<email address hidden>) on branch: master
Review: https://review.openstack.org/195438

[Sean Dague (sdague)]

This devstack bug was last updated over 180 days ago, as devstack
is a fast moving project and we'd like to get the tracker down to
currently actionable bugs, this is getting marked as Invalid. If the
issue still exists, please feel free to reopen it.