Need to test instance booted from volume and exercise ceph-access relation

[Frode Nordahl]

Starting OpenStack Ocata, the default mode of operation of OpenStack Dashboard when launching an instance is to launch it from a bootable volume. At the same version a new ceph-access relation between cinder-ceph and nova-compute is required for this to actually work.

We need to exercise this in the Mojo Specs to catch any regressions.

Tasks:
1) Ensure presence of ceph-access relation between cinder-ceph and nova-compute in all bundles for OpenStack Ocata and onwards
2) Extend helper/mojo_os_utils.py boot_and_test function to optionally support boot from volume
3) Add a step for launching instance that boots from volume to tests/simple_os_checks.py