adding UTCDateTimeField, which always returns datetimes with timezone UTC

[mhrivnak]

re #1254
re #1267

https://www.redhat.com/archives/pulp-list/2015-November/msg00044.html

[mhrivnak]

Looks like the F23 tests are failing for unrelated reasons.

[seandst]

LGTM, because...

• Explicit is better than implicit. (datetime objects returned by mongo are always tz-aware UTC)
• Simple is better than complex. (we aren't messing with timezones, just one timezone)
• In the face of ambiguity, refuse the temptation to guess. (naive datetime objects are ambiguous, tz-aware datetime objects are not)
• There should be one-- and preferably only one --obvious way to do it. (use UTCDateTime)

:)