Be explicit about node_prepare.sh args.

If $HOSTNAME is not set when calling prepare_node.sh then SUDO value
could be confused for the hostname. Quote $HOSTNAME so it is clear which
arg belongs to each arg position in prepare_node.sh.

Change-Id: I9efffa5225de618b57980c1d60da24d7904cd91b
Merge "Be explicit about node_prepare.sh args."