Fix Neutron core_plugin selection and NSX_OVS installation

* Add if statement for NSX
* Fix a string substitution in plugin_neutronnsx/manifests/alter_neutron_server.pp
* Run plugin_neutronnsx::install_ovs in Stage 'netconfig' to avoid circular dependancies

Change-Id: Iac46b79dae54a90b96ae8af7709a5d5676e7e95b
Closes-bug: #1358255
Merge "Fix Neutron core_plugin selection and NSX_OVS installation"