Add new device configuration options to ignore existing configuration at startup
Add a new keep-configuration device configuration option, set to yes by
default. When set to no, on startup NetworkManager ignores that the
interface is pre-configured and doesn't try to keep its
configuration. Instead, it activates one of the persistent
connections.
Also, add a allowed-connections to specify a list of
connections that are allowed to be activated on the device. It is
particularly useful also to forbid some connections, for example those
created by the initrd generator.