factor out common node management tasks

These all sort of touch eachother's context, so just
commit them as one.

1. create common target
2. include passphrase-less ssh key and install public part
   on the common target.
3. node sets hostname on first boot to
   $HOST_BASE-$LAST_2_MAC_BYTES
4. this can then be resolved via avahi (requires
   avahi-daemon on target)
5. add helper functions nssh, nscp, and nenum to operate
   on nodes.
add mr3020 target