This also ensures that we own a reference to the
NMPlatform, NMRouteManager and NMDefaultRouteManager
instances. See bug rh#1440089 where we might access
the singleton getter after destroing the singleton
instance of NMRouteManager. This is prevented by
keeping a reference to those instances -- indirectly
via the netns instance.

Later, we may add support for multiple namespaces. Then it might
make sense to swap the NMNetns instance of a device when moving
the device between namespaces.

Also, drop the use of singelton instances.

https://bugzilla.redhat.com/show_bug.cgi?id=1440089