LU-12761: tests: make version_code() accept two number versions too

LU-12761: tests: make version_code() accept two number versions too

There's now a user in sanity test 103a that calls version_code with
2.6.  Andreas rightfully points instead of fixing the caller we can
just update the code to accept this usage.

Change-Id: I5915cd08a36946c6d26f2e231aa7a820a3eef46a
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36275
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>