Pluggable resource discovery for agents

Precursor-to: BP decoupled-source-sink-discoverable-resources

Add the concept of per-AgentManager resource discovery extensions
loaded via stevedore entry points. The extensions are loaded from
a single namespace:

  ceilometer.discover

shared by all agents. An agent may have a list of per-agent default
discovery extensions associated with it on creation. This is used
by the compute agent, which currently uses a single discovery extension
for all matching pipelines.

The per-pipeline statically configured resources remain in place to
potentially override the per-agent discovered resources - these will
be augmented by per-pipeline discovery in a subsequent patch.

Recast the compute agent's interrogation of the nova servers API
in the form of a discovery extension.

Unify the pollster abstraction used by the central and compute
agents (which had previously diverged along with the introduction
of per-pipeline resources).

Unify the polling task logic used by the central and compute
agents (so that the compute agent now iterates over each pollster
for all local instances, as opposed to interating over all
pollsters for each local instance).

Change-Id: I625df47231a1dbf7ef721c297701799b224ce67e
Merge "Pluggable resource discovery for agents"