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
diff --git a/setup.py b/setup.py
index dc5812f..db7a715 100644
--- a/setup.py
+++ b/setup.py
@@ -73,7 +73,7 @@
     include_package_data=True,
     entry_points={
         "console_scripts": [
-            "osm-mon-exporter = osm_mon.cmd.exporter:main",
+            "osm-mon-prometheus-exporter = osm_mon.cmd.mon_prometheus_exporter:main",
         ]
     },
     dependency_links=[