webadmin: add meta tags to discover dCache version

In the future, the webadmin functional testing may need to know which
dCache version it is testing.  This patch provides this information
in the somewhat standard generator meta-tag in addition to the
easier-to-parse version meta-tag.

Target: master
Requires-notes: yes
Requires-book: no
Patch: https://rb.dcache.org/r/7428/
Acked-by: Albert Rossi