Card origin_broker_136: Replace curl with httpclient when downloading cartridges

This pull request aims to replace the 'curl' command which we are using now to download cartridges
with a 'httpclient' library.

The issue was originally described in this RFE: https://bugzilla.redhat.com/show_bug.cgi?id=1015187

The original problem was to support HTTP proxy when downloading cartridges, which this pull request allows as well.
Two new configuration variables are available for broker.conf:

`HTTP_PROXY` set the HTTP proxy server used for downloading cartridges
`CONNECTION_TIMEOUT` to address the issue reported here: https://github.com/openshift/origin-server/issues/3546