Revert "Revert "ceph-disk: change get_dmcrypt_key test to support different cluster...

Revert "Revert "ceph-disk: change get_dmcrypt_key test to support different cluster name""

This reverts commit d3409d456233f7bc1c0ed23dbc2db29627d33d5c which
reverted 1c747473600a9a7f528ad71a889c35d85ac115b1. Although in the same
pull request, 1c747473600a9a7f528ad71a889c35d85ac115b1 was a fix for
d98d4dbc2741295723f0a316b5bf81f07b140041. The two should have been
squashed but they were not.

When reverting the pull request I reverted all the commits in the pull
request but I did it in the wrong order and the fix became the
problem. Luckily the outcome is a noop instead of a bug, reason why
it was undetected by the tests.

Signed-off-by: Loic Dachary <ldachary@redhat.com>