click >= 7.0 breaks dciclient tests with python2
click >= 7.0 breaks dciclient tests with python2

In our unit-tests we're invoking the CLI through click.testrunner,
passing credendials as environment variables all equal to "".

The code that verifies credentials raises an error if credential pairs are None.

In Python2, click >= 7.0 transforms the empty string for this
environment variables to None, preventing our tests from running.

Also, EL7 still has click 6.x

Change-Id: Ic595fc9860682a4646d313d79bcd58a6dcea0a75

redhat-cip/python-dciclient,redhat-cip/python-dciclient