Add Current build status to Conditional BuildStep

Add support for the Current build status condition.

Also fix the documentation regarding the conditional-step builder name
which requires an hypen and not an underscore as previously stated.

Change-Id: I6ede20c08c42d72d1ccf71c8fe6d3447d3acea90
Reviewed-on: https://review.openstack.org/32292
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins