HUE-670 [core] Hue should use connection pool to talk to webhdfs

Added requests and requests-kerberos packages
HUE-1631 [oozie] Support JobTracker HA in workflows
[beeswax] Update impersonation settings

If SASL, its gets the real username through the protocol and not from the username
of the session.
In case if impersonation, the username of the proxy user needs to be hue and
the proxied username needs to be added as a configuration parameter.