When the change to add support for extended ethertypes was introduced,
one scenario for backwards compatibility was not addressed in the
validator: using the canonical string names for ethertypes. This was
previously supported in the original implementation, using 'IPv4' or
'IPv6' for example.
Since the list of canonical ethertype names is available from os-ken we
consume it from there, which adds an additional dependency for
neutron-lib.
Related-Bug: #1838473
Change-Id: I53c6538dfbeea9691d95c6c555f0c56ae13d1a33
24aef1398b