Update role metadata

This role is now supported on Xenial as of
I78c8c3be9d00fc505fabc83de36dfd43b8e85848.
Update the role's metadata to reflect that.

Also make apt_package_pinning a conditional dependency based on the
host's package manager.

Change-Id: I7fb4b55e4582b2da3f8aab4249597aae15f1bf90

meta/main.yml

@@ -23,6 +23,7 @@ galaxy_info:
     - name: Ubuntu
       versions:
         - trusty
+        - xenial
     - name: EL
       versions:
         - 7
@@ -32,4 +33,6 @@ galaxy_info:
     - development
     - openstack
 dependencies:
-  - apt_package_pinning
+  - role: apt_package_pinning
+    when:
+      - ansible_pkg_mgr == 'apt'