Add cli_dir to tempest config

Add the cli_dir option to the generated tempest config.  This is
required on platforms such as RHEL where the binaries are not in the
standard location.

Note that to be fully operational this requires [1] in tempest so that
the cli path is created correctly.

[1] https://review.openstack.org/#/c/34302/

Change-Id: Ic7369193f1434974366cfabc7550e8545de244cd