The string defined by user to fill in the
option 82 verbose format. If the value of
hh3cDHCPROption82IfFormat is 'normal', or the
value of hh3cDHCPROption82IfNodeType is 'mac'
or 'sysname', it is set with a null string
automatically and can not be modified by user.
It must be set with a non-zero length string
when the value of hh3cDHCPROption82IfNodeType
is set with 'userdefine'.