vyatta-cfg-system: allow password reset for users other than vyos

The password reset option only allows resetting the password for the
default admin (vyos) user.  This now allows the user to be specified
for the password reset, defaulting to the 'vyos' user with a password
of 'vyos'.

Bug #342 http://bugzilla.vyos.net/show_bug.cgi?id=342