Since the restriction on ethertypes is being raised so that ethertypes
other than IPv4 and IPv6 can be specified in security group rules, add a
validator to ensure that ethertypes will be a valid two octet value.
The ethertype handling is configurable so make sure the validator
handles both possible configuration alternatives: the original style
where only IPv4 and IPv6 are valid, and the new style where any valid
two octet number (or it's string equivalent) works.
Related-Bug: #1832758
Needed-By: https://review.opendev.org/670203
Change-Id: I900f370f1de20b5137f138e54c5eec2102f77cce
db04334582