plugin balancer support

This pull-request is designed to add the configuration of the balancer plugin to the puppet-trafficserver module.
This plugin adds load-balancing feature to Apache Traffic Server (ATS).
Official documentation : https://trafficserver.readthedocs.org/en/latest/reference/plugins/balancer.en.html

Balancer plugin is a "remap plugin", and this sort of plugin should not be added to ATS config file "plugin.config". So it can't be handled with the puppet "plugin" class.

Changes :
- remap.config.erb updated to add the balancer configuration
- install parameter added, default to true. To disable ATS installation, set to false. I had to add this parameter to be able to compile ATS on Debian, the package is outdated.
- README updated