gitlint: verify full name in signed-off-by line

Make sure we get full name in the commit message and not the username or
some incomplete data that we need to decipher. We still need to verify
the commiter name that is not part of the commit message body, but this
is out of scope of gitlint.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>