enable microservice

move implementation specific configurations in server.py to
v2/reference module. Making the server.py only responsible
for loading dispatchers which are configured in monasca.conf
file. This change will enable any dispatcher to be loaded
according to the configuration file. For example, to config
two dispatchers to be served on a server, a monasca.conf
file can look like the following:

[DEFAULT]
dispatcher = v2_ref_metrics
dispatcher = v2_ref_alarms

If all the reference dispatchers should be served out of
one server, then the configuration file may look like this:

[DEFAULT]
dispatcher = v2_ref_metrics
dispatcher = v2_ref_alarms
dispatcher = v2_ref_alarm_definitions
dispatcher = v2_ref_events
dispatcher = v2_ref_transforms
dispatcher = v2_ref_notifications

One can use the configuration to load any dispatchers which
may be developed by third party. This way, to load new
dispatchers, one only needs to change the configuration file.

This patch set makes the API server a true miscroservice
server.

Change-Id: I87005f8ff4807e4c818057f1e7866001482a50e1