docker-registry: Trim dependencies now that some storage backends are external

The swift backend moved out-of-core in f700ecda (s3 / boto
support - updated drivers, tests, mock, etc etc, 2014-05-15).

The glance backend moved out-of-core in commit 2e28ad0c (Cleanup,
2014-05-15), which also removed the keystoneclient dependency.

pyopenssl (Debian's python-openssl) was never imported anywhere, and
the Dockerfile reference was removed in 365b6313 (Cleaned-up docker
file, 2014-05-19).