fix TypeError: module object is not callable for cinder.py

There is a bug in def totimestamp when datetime is called, the error is:

<pre>
Traceback (most recent call last):
  File "/usr/bin/oschecks-check_cinder_api", line 6, in <module>
    from oschecks.cinder import check_cinder_api
  File "/usr/lib/python2.7/site-packages/oschecks/cinder.py", line 66, in <module>
    class Novautils(object):
  File "/usr/lib/python2.7/site-packages/oschecks/cinder.py", line 81, in Novautils
    def totimestamp(dt=None, epoch=datetime(1970, 1, 1)):
TypeError: 'module' object is not callable
</pre>

This commit fix the error