Disable Cgsnapshot APIs by default

Currently consistency group APIs are disabled by default because there are
only a very small number of drivers that have implemented them. In
cinder/etc/cinder/policy.json, there are the following entries:

    "consistencygroup:create" : "group:nobody",
    "consistencygroup:delete": "group:nobody",
    "consistencygroup:get": "group:nobody",
    "consistencygroup:get_all": "group:nobody",
    "consistencygroup:create_cgsnapshot" : "",
    "consistencygroup:delete_cgsnapshot": "",
    "consistencygroup:get_cgsnapshot": "",
    "consistencygroup:get_all_cgsnapshots": "",

Note that Cgsnapshot related APIs are not disabled by default. They should be
disabled to be consistent. So the policy entries will be changed to the
following in this patch:

    "consistencygroup:create" : "group:nobody",
    "consistencygroup:delete": "group:nobody",
    "consistencygroup:get": "group:nobody",
    "consistencygroup:get_all": "group:nobody",
    "consistencygroup:create_cgsnapshot" : "group:nobody",
    "consistencygroup:delete_cgsnapshot": "group:nobody",
    "consistencygroup:get_cgsnapshot": "group:nobody",
    "consistencygroup:get_all_cgsnapshots": "group:nobody",

Note: If a driver has CG functions implemented, the CG APIs have to be enabled
by changing "group:nobody" to "" for the CG functions to work.

Change-Id: If59b3da1e84e25e1f547793638c58966e10f488c
Closes-Bug: #1393652
Merge "Disable Cgsnapshot APIs by default"