Add HEAD api response for test s3 server BucketHandler

Current nova test s3 server lacks HEAD for BucketHandler. And it becomes
an issue for Boto after version 2.25.0 [1], which changes underlying
implementation of get_bucket method from GET to HEAD request.

This change fixes this gap by adding HEAD response to test s3 server.
And also added cases for testing bucket existence per Boto document
suggestion[2], which applies for both Boto prior to or after version
2.25.0

[1] http://docs.pythonboto.org/en/latest/releasenotes/v2.25.0.html
[2] http://docs.pythonboto.org/en/latest/s3_tut.html#accessing-a-bucket

Change-Id: If992efa40f7f36d337d1b9b1f52859aa0ae51874
Closes-bug: #1277790
(cherry picked from commit 033f3776c4bc6d0db14b1b9da7c24e207e9628ab)