Throw InvalidArgument on bad Default Gateway

If the Default Gateway parameter is not valid, throw the
InvalidArgument error.

Resolves openbmc/openbmc#3261

Tested: See the InvalidArgument error on values such as
        "blah" or "1.0.8".
Change-Id: Ieeb0da7e3c3806144634cda6cd069758dba92a47
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
1 file changed