gerrit: Allow sending data with ssh command
gerrit: Allow sending data with ssh command

Some commands like git-upload-pack expect data on stdin. These
commands block further execution resulting in a blocked thread. This
adds the possibility to optionally provide data for stdin of the
command.

Change-Id: I21fa2cb310a0472845d216da860e7ce06bd17825

gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,wikimedia/integration-zuul,wikimedia/integration-zuul,gooddata/zuul,gooddata/zuul,gooddata/zuul