Support for multiple KMS instances with selection by current load

Summary:
The load is simply the current number of webrtc connections.
This has mostly been based on removed code from
883d686542f2c72c86b8365978cadf2b13d315e4 and
e400a90942f7dcc160ab45ef0272157774dc59c5.

Verified that with two KMS instaces and two rooms, each get's used
once.
Look for "KMS less loaded is ws://localhost:8889/kurento with a load of
0.0" in the logs.

Reviewers: vanmeeuwen, machniak

Differential Revision: https://git.kolab.org/D2080