Fix unit test failure with python3.6
Fix unit test failure with python3.6

Since python3.6 os.path.join can be either bytes or
a str. So mock os.path.join in order for the unit tests to
pass.

Change-Id: I82c793875d888092e5c814727a6c4ad4053e76fb
Signed-off-by: Chuck Short <61c7e57c8f71fbf1f6c3fcd85c16ccd0f494e116@redhat.com>

ArchiFleKs/magnum,ArchiFleKs/magnum,openstack/magnum,openstack/magnum