Create a single decorator for API methods to be used be accross all
synnefo. This decorator does the following:

* Proper logging of 5xx faults and unexpected errors
* Authentication with Astakos using snf-astakos-client
* Sets proper HTTP response and cache control headers

Also move some common functions from various apps to
'snf_django.lib.api.utils'

Refs #3358 #3448