Change ownership of {{ install_lib_dir }}/openstack_dashboard/local

When {{ install_lib_dir }}/openstack_dashboard/local is owned by root,
horizon fails to load due to a LockFailed exception being raised due to
horizon's inability to write to
{{ install_lib_dir }}/openstack_dashboard/local.  Changing the
ownership to www-data:www-data fixes this.

If we have any further issues, we may need to chown
{{ install_lib_dir }}/openstack_dashboard/ entirey to
www-data:www-data.

Closes issue #362