Adapt version.py to dates for incremental builds
Adapt version.py to dates for incremental builds

To allow developer builds to upgrade cleanly, move
away from the l-d-t style of tag+commit_hash to tag+date
as this allows smoother upgrades once the code has been
packaged.

Change-Id: I40467441a5df090b4949bd813c30000dc9fa6edb

Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server