Allow murano-agent to be disabled

In some circumstances murano-agent isn't required (e.g. in
environments where heat SW config is capable alone of performing
configuration). In this case it's not necessary to have the
additional overhead of rabbitMQ connections for the AgentListener
that will never receive a message.

Patch adds a config option 'disable_murano_agent' that no-ops
AgentLister.start() and raises an exception on Agent._send()

Change-Id: I565caaae21925c48f2a0adea18036239cac91c77
Implements: blueprint disable-murano-agent
Merge "Allow murano-agent to be disabled"