Missing dependencies in Ubuntu Linux bootstrap

We seem to be missing some dependencies in the Ubuntu Linux bootstrap. Those I have noticed so far:
- subversion
- OpenGL stuff which leads to:

```
Recipe 'gst-plugins-bad-1.0-static' failed at the build step 'post_install'
Traceback (most recent call last):
File "./cerbero/build/oven.py", line 143, in _cook_recipe
stepfunc()
File "/home/_username/cerbero/recipes/custom.py", line 93, in post_install
shutil.copyfile(src, dest)
File "/usr/lib/python2.7/shutil.py", line 82, in copyfile
with open(src, 'rb') as fsrc:
IOError: [Errno 2] No such file or directory: '/home/_username/cerbero/sources/linux_x86_64/gst-plugins-bad-1.0-static-1.5/static-build/opt/openwebrtc-0.3/lib/gstreamer-1.0/libgstopengl.a'
```

Start by inspecting the Ubuntu Linux bootstrap and the dependency handling around cerbero/build/oven.py and recipes/custom.py, then run the bootstrap to reproduce the missing subversion/OpenGL dependency failure. Done means the required dependencies are available and the gst-plugins-bad-1.0-static build completes without the reported missing libgstopengl.a error.

Written by the indexing model from the issue text.