Move RobotFramework dependencies to csit job

Move RobotFramework dependencies to csit job

Dependencies:

- Jenkins Robot Framework plugin 1.6.1
  https://issues.jenkins-ci.org/browse/JENKINS-29178

Changes:

- Remove java, git, and perl-XML-XPATH install command as it's already
  covered by the baseline image.
- Only install the minimal amount of packages to get python virtualenv
  going.
- Use virtualenv to setup robotframework at the job level. (Further
  improvement to this could be to use a JJB variable to allow projects
  to define which python modules they need in their project file)

This work is needed to make maintenance of python modules simplier and
move them out of the vagrant / spinup scripts and directly into the job
itself. This work is necessary to allow us to upgrade to CentOS7 for our
robotframework slave.

Change-Id: Ib167e5ba2989269df177ec070811c83618c25cb6
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>