Add a hook so additional scripts can be run after creating VM

Allow additional scripts to be run as part of the OMERO Virtualbox build.

For example, to include OMERO.searcher as part of the VM (note this is using my own repo/branch for convenience):
```
PARENTDIR=$PWD
git clone git@github.com:manics/omero_searcher.git
cd omero_searcher
git checkout origin/merge_all
```

The in the openmicroscopy checkout:
```
export OMERO_POST_INSTALL_SCRIPTS=$PARENTDIR/omero_searcher/docs/vm/setup_omero_searcher.sh
./build.py build-vm
```

http://trac.openmicroscopy.org.uk/ome/ticket/11240

Note the actual OMERO.searcher VM setup script is a bit of a mess due to the ancient pip version which is missing a lot of functionality.