Use feature detection to determine whether AZ should be selectable

[mansam]

https://bugzilla.redhat.com/show_bug.cgi?id=1549128

This PR is part of the implementation to allow choosing an availability zone when creating a new Openstack Cinder volume. It makes the existing AWS-specific AZ selection code generic, and allows it to be enabled by checking the supports_volume_availability_zones feature. Depends on the core PR which adds supports_volume_availability_zones.

[miq-bot]

Checked commit mansam@427a2b5 with ruby 2.3.3, rubocop 0.69.0, haml-lint 0.20.0, and yamllint 1.10.0
3 files checked, 2 offenses detected

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• 💣 💥 🔥 🚒 - Linter/Haml - Linter::Haml STDERR:

warning: parser/current is loading parser/ruby23, which recognizes
warning: 2.3.8-compliant syntax, but you are running 2.3.3.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.

spec/controllers/cloud_volume_controller_spec.rb

• ❗ - Line 261, Col 49 - Layout/AlignArguments - Align the arguments of a method call if they span more than one line.

[mansam]

This PR replaces #5155