Handle rename of the marionette-client package to marionette-harness

[EricRahm]

From dev-platform:

Upcoming rename of the marionette-client package to marionette-harness
Henrik Skupin via lists.mozilla.org

to tools-marionet., dev-platform
Hi,

Please note that we will soon rename the marionette-client Python
package to marionette-harness. This work is currently happening on bug
1320073. The reason for it is that there is lots of confusion between
the package names and the folder names in-tree:

testing/marionette/client -> marionette-driver
testing/marionette/harness -> marionette-client