Refactor: cts: Simplify cts-lab --cib-filename processing

"not self.env['CIBfilename']" means either None or empty string. It
wouldn't make sense to try to manipulate a file whose name is the empty
string. So update clustermanager.py for clarity and idiomaticity.

Use a ternary for setting self["CIBfilename"] in the first place.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>