commands: log HTTP req/resp info

Since everything goes through swiftclient, we leverage the swiftclient
logging to do this.

This includes also a custom log filter to squelch some annoying ERROR
messages that raised if a container GET fails with a 404.

This fixes https://github.com/zerovm/zpm/issues/131.
test_commands: add a test for SwiftLogFilter
Merge pull request #171 from larsbutler/better-req-resp-logging

Better HTTP request/response logging