Rack controller should confirm when connecting to a different MAAS

[Lee Trager]

A MAAS user has a MAAS 2.6 environment. The MAAS 2.6 environment has a secondary rack controller which was also being used as a libvirt KVM host. They created a new MAAS 2.9 environment in parallel with their MAAS 2.6 environment. The user mistakenly thought that they could connect a MAAS 2.6 rack controller to both the 2.6 and 2.9 environment at the same time. This caused strange issues in MAAS such as the VMs being deleted from the filesystem yet still present in the MAAS 2.6 database.

1. maas-rack register should confirm when being disconnected from one MAAS and registered with a new MAAS environment.
2. maas-rack register should verify the version of MAAS it is connecting to has the same major/minor. MAAS version information is available anonymously at http://$IP:5240/MAAS/api/2.0/version/
3. An explicit log entry should be added to rackd.log that the rack was switched from one MAAS to another.
4. A rack which is also a VM host should be not be able to register with a different MAAS

[Jerzy Husakowski (jhusakowski)]

Added a comment to the specification for new rack/region architecture to handle it there.