vyatta-cfg-system: add complete user config entry on password reset

Add the default plaintext-password entry to the default vyos user
created during password reset.

Bug #342 http://bugzilla.vyos.net/show_bug.cgi?id=342
vyatta-cfg-system: set default vyos password hash to sha-512 when reset

In line with Bug #318, set the default vyos user password hash to use
SHA-512 instead of MD5 when recreating the user during a password reset

Bug #355 http://bugzilla.vyos.net/show_bug.cgi?id=355