Add support for multiple policy files

Most of the neutron plugins provide an updated version of policy.json
file with full list of rules, but at the same time there are a lot of
other plugins which provide their own policy files and store them in
the policy.d/ folder: neutron-fwaas, networking-bgpvpn, vmware-nsx,
ect...
To implement the tests for such plugins the Patrole should be able to
load and merge multiple policy files for any of the services.

Modify the discover_policy_files function to discover all policy files
for each of the services. Using glob.glob() function makes it possible
to use patterns like '*.json' to discover the policy files.
Modify the _get_policy_data function to load a data from all discovered
policy files for a service.

Update the unit test according to the changes.

Change-Id: Ib24f3d6d7a5ffdeaecce579af9795fd897dce872
diff --git a/releasenotes/notes/multiple-policy-files-9aa7f7583283739e.yaml b/releasenotes/notes/multiple-policy-files-9aa7f7583283739e.yaml
new file mode 100644
index 0000000..a3555e6
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+++ b/releasenotes/notes/multiple-policy-files-9aa7f7583283739e.yaml
@@ -0,0 +1,17 @@
+---
+features:
+  - |
+    In order to implement the tests for plugins which do not maintain the
+    ``policy.json`` with full list of the policy rules and provide policy file
+    with only their own policy rules, the Patrole should be able to load and
+    merge multiple policy files for any of the services.
+
+    - Discovery all policy files for each of the services.
+      The updated ``discover_policy_files`` function picks all candidate paths
+      found out of the potential paths in the ``[patrole].custom_policy_files``
+      config option. Using ``glob.glob()`` function makes it possible to use
+      the patterns like '\*.json' to discover the policy files.
+
+    - Loading and merging a data from multiple policy files.
+      Patrole loads a data from each of the discovered policy files for a
+      service and merge the data from all files.