avoid renaming --boot to --sync

That broke updating existing systems, since --boot is what makes the remote
propellor update itself.

The hostname is no longer needed by --boot for new propellor's, but is
still passed for old ones.

Note that there will be a double run of propellor when upgrading via
--spin, because it now runs --boot followed by --spin on the remote host,
and the old --boot also satisfied all properties.
propellor spin