Need a mechanism to disconnect a NIC from a dead switch

[henn]

If a HIL-managed switch dies, it won't be possible to detach the node NICs from it since their networks won't be able to be disconnected as the switch is dead.

To fix this, it might make sense to allow a "--force" option to detaching a NIC, which leaves the networks in place so that the cables can be moved to a new switch. We can then add a call to revert_port()/reset_port() to port_connect_nic(), which will configure the port correctly.

This is complementary to #761, and maybe should be done at the same time.