Move policy check of start/stop to api layer

This patch moves "compute:start/stop" to api layer for ec2 and nova
v2 api, and uses "compute:v3:servers:start/stop" for nova api v3.
Also removes policy check from compute api. And give default rules
for v3 policy check.

Add core_authorizer with 'compute:' as the scope of core api in v3.
That's for distinguishing core and extension api. Use different namespace
for policy. The policy of core api will be 'compute:v3:extension_name'.

Partially implements bp v3-api-policy
DocImpact

Change-Id: I6c4f1c677b64015c61440f9db941d1ab9cf77b48
Merge "Move policy check of start/stop to api layer"