LSI Correlator: Use virtual env

As a developer I would like to be able to manage the python dependencies of IBEX. As is documented [here](https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/Python-dependencies) we have decided that it would be easier to manage dependencies if every python project had it's own virtual environment. The LSI correlator contains two dependencies, `twisted` and `autobahn` that we know are not required anywhere else in IBEX. If we move the correlator to the new model we could remove these as dependencies form the standard python installation.

# Acceptance Criteria
* The LSI correlator has a `requirements.txt` listing it's requirements (other than `genie_python`
* The LSI correlator is run in a virtual environment
* Any requirements that are unique to the correlator are removed from the standard python install