Conditionalise automatic enabling of disabled host

The change Ib8d67838ceb73c5b1cdc9498c17b335e9e5bb6f3 introduced
automatic behaviour to the nova-compute hosts, which will now
automatically disable themselves if they lose contact with libvirtd,
and re-endable themselves on a successful reconnection.

However, this tramples over any operational service disablement
that may be in place (on an otherwise working host) for any number
of good reasons.

As suggested in bug #1250049, this is a quick fix which attaches
a prefix to any host disable_reason string, and only re-enables
a host if that string is still present.

Fixes-bug: #1250049
Change-Id: Ia7544a357d1385db5fa92e500cb691f49436cfc9
Merge "Conditionalise automatic enabling of disabled host"