Add SIGHUP handlers to reset RPC version pins

Add SIGHUP handlers to reset RPC version pins

Adding SIGHUP handlers (by implementing reset from oslo.service) to
cinder-scheduler, cinder-backup and cinder-volume that reset cached RPC
version pins. This is to avoid the need to restart all the services when
upgrade of the deployment is completed.

Some changes go a little deep into the stack, because to reload all the
pins we need to recreate <service>.rpcapi.<service>API objects that are
stored in the memory.

Please note that SIGHUP signal is handled by oslo.service only when
service runs in daemon mode (without tty attached). To test this commit
in DevStack you need to add "&" to the end of the command that starts
the service.

Situation is more complicated with the API service, so we're leaving it
with restart required for now. In the deployments with HA cinder-api is
typically behind a load balancer, so restarting individual nodes
one-by-one should be easy.

DocImpact: Add information on rolling upgrades procedures to the docs.
Implements: blueprint rpc-object-compatibility

Change-Id: I03ed74e17dc9a4b9aa2ddcfbeb36a106a0f035f8