Before this change if master node had been upgraded to Fuel of
versions >= 8.0, attempt to re/deploy existing clusters would have failed
as attributes of those clusters would have lack 'deployed_before' field.
Since processing of the flag is temporary solution it should not be
coded in data base migration logic. Thus the change introduces creating
of the attribute with given value if it is not already present.
Change-Id: I302ddb340d2ecf14b1fa414fda71aa7f32363bdf
Closes-Bug: #1551339
cfa146e9dd