customization: existing storage domain detection

Detect existing storage domain and fill environment
variables without asking to the user.

Moved the vdsmd service start at late setup stage and
restart at misc stage after its configuration for
allowing the storage domain detection.

Moved vdsmd readyness check from storage to vdmsenv plugin

Change-Id: I9016357349a357b8a147b00fc46cf1a70604168a
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>