Implemented HA mode for metadata repository

Metadata Repository will replicate all API calls which modify the data
on drive to the slave nodes. This will eventually synchronize the data
on the master node with the data on the slaves.
Master/slave selection is out of the scope: the proper node will be
selected by pacemaker.
The node which receives a replicatable call without special header
considers itself as primary and will asynchronously resend the call to
all other nodes, considereing them as slaves. These calls will have
special headers set, indicating that those nodes are slaves and should
not replicate data.

Same commit adds config sample, as the config file modification is
required to enable HA

Change-Id: Ia34e66c3a879b92553ca9c9a54ecec39d911aace
Implements: blueprint murano-repository-ha