Allow access to more clouds files

There are more options to specify a clouds.yaml file:

a) the file can end with .yml, .yaml or .json
b) the file can be located in the user home dir or in /etc/openstack

So allow read-only access to /etc/openstack and ~/.config/openstack/ .

See
https://opendev.org/openstack/openstacksdk/src/branch/master/openstack/config/loader.py#L49

Change-Id: I84276ff4b12acc5f4ac5092580bb0e652d518e54
(cherry picked from commit 528db56bbcd0bd57916ae97b1cfc3d6f548b721a)

snapcraft.yaml

@@ -17,10 +17,14 @@ grade: stable
 base: core18
 
 plugs:
-  dot-config-openstack-clouds-yaml:
+  dot-config-openstack:
     interface: personal-files
     read:
-    - $HOME/.config/openstack/clouds.yaml
+    - $HOME/.config/openstack
+  etc-openstack:
+    interface: system-files
+    read:
+    - /etc/openstack
 
 apps:
   openstack:
@@ -29,7 +33,8 @@ apps:
     plugs:
       - network
       - home
-      - dot-config-openstack-clouds-yaml
+      - dot-config-openstack
+      - etc-openstack
       - ssh-public-keys
   glance:
     command: bin/glance