Simplify logic to ensure absolute path
Simplify logic to ensure absolute path

We can simplify the conditional logic to ensure a path is absolute, by
using os.path.join instead of string concatenation.

Change-Id: I3d2441bbb235fef4775d4bc481574d85866a7fb2

wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul