Replace some unnecessary os.path.join()
Replace some unnecessary os.path.join()

When using pathlib, it's not necessary[1] to use in conjunction with
os.path.join(). This change just simplifies some of the use of
pathlib within tripleoclient.

We may want to expand our use of pathlib in the future to provide
additional flexibility over the traditional os.path interfaces.

[1] https://docs.python.org/3/library/pathlib.html#operators

Change-Id: I8e2a321cdaeea75dd86635de92721a1515f63f4f

openstack/python-tripleoclient