Enhance Ceilometer API Role Based Access Control in Preparation for Keystone v3 API

Current access control for the API is all or nothing. A policy.json file is used but it simply determines if the user is admin, granting all privileges, or scoping for a single project.

With upcoming Keystone v3 enhancements we can expand the granularity of access control to allow cross-project access by non-admins. This functionality will be useful for admins of groups of users/tenants who should not be the admin of the entire system.

We will accomplish this by using a decorator on the API functions. The decorator will control access based on user/tenant roles and rules specified in the policy json file.

Example of decorator usage in v2 reporting API:
    @wsme_pecan.wsexpose([OldSample], [Query], int)
    @rbac_validate.protected('meters')
    def get_all(self, q=None, limit=None):
        """Return samples for the meter.

        :param q: Filter rules for the data to be returned.
        :param limit: Maximum number of samples to return.
        """
Decorator contains code similar to what's in acl.py already, but sets project/users here instead of various places within the v2.py code:

def protected(controller_class):

    global _ENFORCER
    if not _ENFORCER:
        _ENFORCER = policy.Enforcer()

    def wrapper(f):
        @functools.wraps(f)
        def inner(self, **kwargs):
            pdb.set_trace()
            self._rbac_context = {}
            if not _ENFORCER.enforce('context_is_admin',
                                     {},
                                     {'roles': pecan.request.headers.get('X-Roles', "").split(",")}):
                self._rbac_context['project_id'] = pecan.request.headers.get('X-Project-Id')
                self._rbac_context['user_id'] = pecan.request.headers.get('X-User-Id')
            return f(self, **kwargs)
        return inner
    return wrapper

acl.py could be deprecated since its only function is to determine if a user is admin, and the decorator accomplishes this.

Example policy expansions:

Current policy.json only verifies user is admin:
{
    "context_is_admin": [["role:admin"]]
}

New rules allow separation of access control by method and expanded roles. Also compatible with Keystone v3 expanded functionality where domains are supported.
{
    "context_is_admin": [["role:admin"]],
    "admin_or_cloud_admin": [["rule:context_is_admin"],["rule:admin_and_matching_project_domain_id"]],
    "telemetry:alarm_delete": [["rule:admin_or_cloud_admin"]]
}