New -connectViaHttpOnly cli client jar command line option.

That option connects via http, without attempting to connect via port.
I.e. tells CLI to connect via http only, skipping connecting via port.
This trace shows in cli client jar command stdout when option is used:
Skipping connectViaCliPort => connectViaHttp

Also includes dout.flush internally proposed by CloudBees people, in
Connection.writeByteArray

This can be useful for when connecting via port makes cli hang, as per
issues.jenkins-ci.org/browse/JENKINS-20709
-even though it is still hard to reproduce such hanging as we speak.