Add a configuration setting to consider new nodes 'installed'

Razor's nodes carry a 'installed' flag that indicates whether the node should be sent through the policy table and therefore possibly reinstalled when it boots.

Adding a configuration setting that marks all newly discovered nodes as 'installed' makes it possible to bring Razor into an environment with already installed machines, even if they are not fully known, and is safe for the user, as nodes will only be (re)installed after the user cleared the 'installed' flag using the 'reinstall-node' command