designate-mdns fails if monasca-statd not installed

[Corey Bryant]

designate-mdns is failing on ocata if monasca-statsd is not installed.

This is without any options set in the [monasca:statsd] config section, therefore statsd is disabled in this case.

systemd[1]: Started OpenStack Designate DNSaaS mdns.
designate-mdns[16241]: Traceback (most recent call last):
designate-mdns[16241]: File "/usr/bin/designate-mdns", line 6, in <module>
designate-mdns[16241]: from designate.cmd.mdns import main
designate-mdns[16241]: File "/usr/lib/python2.7/dist-packages/designate/cmd/mdns.py", line 26, in <module>
designate-mdns[16241]: from designate.mdns import service as mdns_service
designate-mdns[16241]: File "/usr/lib/python2.7/dist-packages/designate/mdns/service.py", line 23, in <module>
designate-mdns[16241]: from designate.mdns import handler
designate-mdns[16241]: File "/usr/lib/python2.7/dist-packages/designate/mdns/handler.py", line 27, in <module>
designate-mdns[16241]: from designate.mdns import xfr
designate-mdns[16241]: File "/usr/lib/python2.7/dist-packages/designate/mdns/xfr.py", line 29, in <module>
designate-mdns[16241]: class XFRMixin(object):
designate-mdns[16241]: File "/usr/lib/python2.7/dist-packages/designate/mdns/xfr.py", line 34, in XFRMixin
designate-mdns[16241]: def zone_sync(self, context, zone, servers=None):
designate-mdns[16241]: TypeError: 'NoneType' object is not callable

It seems the TypeError occurs somewhere within the @metrics.timed decorator:

    @metrics.timed('mdns.xfr.zone_sync')
    def zone_sync(self, context, zone, servers=None):
        ...