Remove httplib2 dependency from ganeti.rapi.client

- It's possible to implement all functionality in ganeti.rapi.client
  using Python's standard modules httplib and urllib2
- By doing so, proper SSL certificate verification is implemented
- Adjust some of the code to Ganeti's code style (this is not yet
  finished)

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>