We now refuse to start more than one monitor on the same machine if
hostnetworking is enabled.
Supporting this creates a lot more complexity from the rook side. Also
having 3 monitors running on the same machine is not a production setup.

This means multi-cluster support on the same machine
is not possible anymore when hostnetworking is enabled.

Resolves: https://github.com/rook/rook/issues/2604

Signed-off-by: Sébastien Han <seb@redhat.com>