Refactor out Ceilometer V2 API related code

As of Queens the Ceilometer V2 API has been removed.  Storage
driver related code in monasca-ceilometer can thus be removed.

Backport some refactoring that came up in creating
https://review.openstack.org/#/c/562400/
Realized some class names could be clearer and fit better with
Ceilometer.

Update test-requirements.txt with Rocky release of
python-monascaclient.  Also include more license info.

Specify py35 instead of py34 in tox.ini (gates already using py35).

Include updates to devstack configuration to keep up with master
ceilometer.  These include adding values for the [monasca] section
in ceilometer.conf and refreshing setup.cfg as well as updating
how the connection url to Monasca API is configured.

Also include some testing notes in devstack/README.md

Change-Id: I9291dfc06d67f18109a0ff66184bd418c6361977

ceilosca/ceilometer/__init__.py

@@ -14,7 +14,4 @@
 
 
 class NotImplementedError(NotImplementedError):
-    # FIXME(jd) This is used by WSME to return a correct HTTP code. We should
-    # not expose it here but wrap our methods in the API to convert it to a
-    # proper HTTP error.
-    code = 501
+    pass