Prevent override 10-globally-managed-devices.conf overwritten by package update (#745)

* Rename NM conf not to overwritten by package update

We overwrite 10-globally-managed-devices.conf that network-manager package
provides when building the iso. However, this conf can be re-overwritten
after installation of OS when network-manager package is updated,
causing NM no longer manages Ethernet devices.

So, rename our conf to a filename that won't conflict with files
provided by network-manager package and make sure it's loaded after and
overwrite 10-globally-managed-devices.conf.

* Create blank override file under /etc instead of creating under /usr

Author: ryonakano