Update role metadata
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

openstack/openstack-ansible-rsyslog_server,openstack/openstack-ansible-rsyslog_server