Adds to manager init_host validation for instances location

While the compute was down the instances running on it could be
evacuated to another host. The method checks that instance host
identical to current host. Otherwise destroying it.

part of blueprint rebuild-for-ha

DocImpact

Change-Id: Ic90df2b2887ee203e6d8261084e3f97773c5d81c
Co-authored-by: Oshrit Feder <oshritf@il.ibm.com>
Clarify the DBApi object in cells fakes

Adds a docstring and makes instance_get_by_uuid() raise by default.
It's only used as a stubbing point, but returning 'None' is not really
valid and having it raise might save someone some confusion in the
future.

Change-Id: Ie3fa333288f9c0263ead69c2d307101b8583dea7
Merge "Clarify the DBApi object in cells fakes"
Merge "Adds to manager init_host validation for instances location"