CA-108916: add call to V6rpc.get_current_edition API

We add this API so that when v6d returns a failure on apply_edition, xapi is
able to find out the correct edition to set itself to. This fixes a "bug"
(design decision, really) where 1) v6d holds a valid license, 2) xapi
apply_edition with bad license-server url, 3) v6d release license, and fails to
check out new license, 4) xapi remains at the current licensed edition, but v6d
doesn't hold the license for that edition.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Merge pull request #1378 from mcclurmc/ca108916-get-current-edition

CA-108916: add call to V6rpc.get_current_edition API