Dynamic policy file discovery

Patrole should eventually support other services like Heat and Murano, not just the Big Tent services included in Tempest.

Part of that requires that Patrole be able to dynamically discover custom policy files. The current solution proposed below is not perfect, but will work in more situations than today.

Dynamic discover of policy files means: Looking in prescribed locations for each service's policy file and using the first one that is encountered.

This means that

    cfg.StrOpt('cinder_policy_file',
               default='/etc/cinder/policy.json',
               help="Location of the neutron policy file."),
    cfg.StrOpt('glance_policy_file',
               default='/etc/glance/policy.json',
               help="Location of the glance policy file.")
    ...

in [0] should be replaced with a single config option called "custom_policy_file_loc" and will look something like:

    cfg.ListOpt('custom_policy_file_loc',
               default=['/etc/%s/policy.json'],
               help="""List of the paths for each custom policy file.
Each policy path assumes that the service name is included in the path.
The paths should be ordered by precedence, with high-priority paths
before low-priority paths. The first path that is found to contain
the relevant policy file by service will be chosen.""")

Then in tempest.conf under [rbac] the following could be included:
custom_policy_file_loc=['/etc/%s/policy.json', 'foo/bar/%s/policy.yaml']

Afterward, discovery of each service's policy file can be included in RbacPolicyParser once, inside ``validate_service`` [1].

The logic here will have to be updated as well: [2].

[0] https://github.com/openstack/patrole/blob/25569dc76b444e1cbed3e612d5b8c35f3cecec31/patrole_tempest_plugin/config.py
[1] https://github.com/openstack/patrole/blob/25569dc76b444e1cbed3e612d5b8c35f3cecec31/patrole_tempest_plugin/rbac_policy_parser.py#L87
[2] https://github.com/openstack/patrole/blob/25569dc76b444e1cbed3e612d5b8c35f3cecec31/patrole_tempest_plugin/rbac_policy_parser.py#L74