Wait for volume attachment in CI before detach

In some cases the volume attachment is not finished by the time we
attempt to detach it, causing the following error:

Volume status must be 'in-use' and attach_status must be 'attached' to detach.

This change waits for the volume to be attached before detaching.

Change-Id: Id23d114cf8a2b40f8f0784cbfb84e9fa0829adc0