bootstrap_lxc_manager: allow command line arguments to modify/add plugin...

Allows this:
`python2.7 cosmo-manager/vagrant/bootstrap_lxc_manager.py --working_dir=$WORKINGDIR --cosmo_version=$VERSION --config_dir=$CONFIGDIR --define_plugin cloudify.plugins.openstack_host_provisioner https://github.com/Fewbytes/cosmo-plugin-openstack-provisioner/archive/feature/CLOUDIFY-2159-openstack-refactor.zip --install_plugin cloudify.plugins.openstack_host_provisioner`