Allow the developer of a watcher plugin to add specific parameters

In Watcher, it is possible to deploy and configure new strategies,actions,planners,ect and associate each "plugin" to a Python class, without the need to redeploy a new version of Watcher.

However, it should be possible to pass some extra parameters depending of the configuration of the openstack cluster or some behaviour ( timeout, weights, ect) on which watcher is operating.

This is the reason why, we should give the possibility to the developer to add some parameters on each plugins.