By disabling nova-compute all instances created or migrated will only land
on maintaned host that will have nova-compute enabled.
In Nova terms disabled nova-compute also means the host is in maintenance.
Story: 2003848
Task: #26641
Change-Id: I8b05de9bd3653187fc210700b5b39faeb2fffc03
Signed-off-by: Tomi Juvonen <tomi.juvonen@nokia.com>