Added support for dynamic transport client ratio in tests.

Currently tests only run with node clients but eventually we want to
run all tests with randomly choosen node / transport clients. To enable
this during development and on test servers as a transition phase this
commit adds the ability to allow a fraction of the clients used in
tests to be transport clients. By default this is still disabled.
To enable transport clients in tests pass '-Dtests.client.ratio=[0..1]'
where '1.0' will force transport clients and '0.0' completely disables
them. If an empty string is passed the ratio is chosen at random for
each test.