As per the new release model, non-SLURP releases
will go directly to EOL (without transition to
unmaintained status) and Tempest and its plugins
needs to release the <series>-last tag for those
branches also.
To do that, allow validate scripts to release
<series>-last tag for EOL series also.
Change-Id: I4735eb7d6c2b54163aa3e8cb276e6d63d182c923