Fix bootstrap script

This PR makes three changes:

1. splits the kernel installation and the requirements for add-apt-repository into two steps
Installing the kernel can fail in vagrant. In my case, it failed to find that kernel.
2. replaces software-properties-common with python-software-properties
$ apt-file search add-apt-repository
python-software-properties: /usr/bin/add-apt-repository
3. adds curl to the list of packages to be installed as dependencies
This was missing in my vagrant VM.