Change warn to debug logs when migration context is missing
Change warn to debug logs when migration context is missing

MigrationContext record missing is currently a regular situation. There
will be no migration record at all (and thus no MigrationContext record)
in case of a rebuild on the same host, and in case of evacuate request,
which explicitly specifies the target host.

With this in mind we demote the log messages that point out the missing
migration context to debug.

Closes-Bug: #1549853

Change-Id: I96b6c859a9d450707f997e43e37f770e2a9dea7a
(cherry picked from commit b70f4320a296bf7111d276bef5e92c289fe5fa8f)

CCI-MOC/nova,Francis-Liu/animated-broccoli,Francis-Liu/animated-broccoli,CCI-MOC/nova