Install the vendored HttpCtrl, not PyPI's 0.3.3

requirements.txt pins robotframework-httpctrl==0.3.3 from PyPI, so the vendored copy in libraries/robotframework-httpctrl/ is never installed: the stub-matching fixes of !282 (merged), !284 (merged) and !311 (merged) (and the open !313 (merged)-!316 (merged)) never run, and the keyword Get Request Url Params, called by six distributed-operation test cases, does not exist in 0.3.3. Now -e ./libraries/robotframework-httpctrl/. The ITB image copies the vendored library next to requirements.txt and installs from /tmp (pip resolves the relative path against the working directory). Checked: a fresh venv and the Dockerfile's steps in a python container both install the vendored 0.3.1 with the keyword.