Update command used to extract version from tag

Fix for issue SPEC-1782

XDS Version is based on git tags and 3 formats are used to tag the same AGL
version, for example: flounder_6.90.0, flounder/6.90.0, tag: 6.90.0.
For time to time (don't know why always reproducible) the "git describe"
command returns the number version/tag (6.90.0) that is expected and
sometimes it's the string version/tag (flounder/6.90.0) that is
problematic.

Change-Id: I9fdbe93cfea917493a8d4e8346a72cc97cf9c5bd
Signed-off-by: Sebastien Douheret <sebastien.douheret@iot.bzh>