Buildbot multi-master and load balancing

Due to the increased number of builders, buildbot needs to run multiple master. However, each master has access to all worker machines, so a load balancing scheme is needed in order to ensure that the worker machines are not overloaded. Since the workers used are latent, that means that there is no process running on each worker machine. So, a way of communicating which each machine is needed in order to get the current load and decide if a new build can be started. Since we already have a Zabbix monitoring in place, it makes sense to use Zabbix in order to get the load.

Step 1. Change the master.cfg to easily support multi master
Step 2. Use the Zabbix Python API to add load balancing.