Support If-[Un]Modified-Since for object HEAD

We already supported it for object GET requests, but not for
HEAD. This lets clients keep metadata up-to-date without having to
either fetch the whole object when it's changed or do their own date
parsing. They can just treat Last-Modified as opaque and update their
idea of metadata when they get a 200.

Change-Id: Iff25d8989a93d651fd2c327e1e58036e79e1bde1
Print 'Container Count' in data base info

Currently, 'Container Count' was missing in data base info.
So this patch will help printing 'Container Count' also.

Change-Id: I1ca80ee79e71b086b30fd2d1ab024ea1cfb324f5
Merge "Support If-[Un]Modified-Since for object HEAD"
Merge "Print 'Container Count' in data base info"
Load constraints from cluster info dictionary

Use constrainst from the new "swift-constraints" section of test.conf,
fall back to those found in the response to the /info API call,
ultimately falling back to the constraints module's effective
constraints.

Change-Id: Iea01c9c4b5148faa10004a240df411cbe7336a6a