Adds use of CustomCollector in Prometheus exporter

Using a CustomCollector gives more control over the collection
process, which allows handling the removal of metrics when a NS
is deleted. Metrics now have a lifespan of a collection cycle, so
metrics of deleted vdus are now not visible in the exporter.

It also adds MonCollector class, which allows to abstract collection
logic and facilitates testing.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: Idfdb86c1ee9facd07187e6582954ae4cab32a5b4