Add method in Agent class to allow message handling extensions
Add method in Agent class to allow message handling extensions

A new method is added to handle message creation separately from
the send logic. This will allow to extend Agent code by using inheritance
so that there is no need to rewrite send method in order to change
message content.

Closes-Bug: #1444547
Change-Id: I95e35974f7df017383df206293e8a7c93af04489

satish-avninetworks/murano,NeCTAR-RC/murano,NeCTAR-RC/murano,satish-avninetworks/murano,NeCTAR-RC/murano,olivierlemasle/murano,DavidPurcell/murano_temp,chenyujie/hybrid-murano,satish-avninetworks/murano,olivierlemasle/murano,DavidPurcell/murano_temp,sajuptpm/murano,DavidPurcell/murano_temp,chenyujie/hybrid-murano,DavidPurcell/murano_temp,sajuptpm/murano,olivierlemasle/murano,NeCTAR-RC/murano,openstack/murano,satish-avninetworks/murano,olivierlemasle/murano,openstack/murano