Check for PRIVATE_VARIABLES config, and load that in addition
to emulabconfig variables; this allows me to put a bunch of passwords into a file instead of elabinelab attributes, which land in log files, mail logs, ns files, etc. Do not set root password to fixed string in rc.mkelab. Instead, if one does not come in the config variables, generate a random password string and write that into a file in /usr/testbed/etc. Localize NAMED_VARIABLES from the config variables if it exists.