Refactor scaling group config and adds partial support for vnf metrics

Adds support for scaling groups based on vdu-metric corresponding to a juju
metric. It also includes modifications to support MON modifications regarding
vnf-monitoring-params.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: I1a1604f965c077fa14a19ad7a26da0facc4818ba