Fewer parameters, one more dependency.

Move out of configuration file and into command line parameters options
which are auto-generated or not possible to provide as string (python's
_ssl module insists on getting key & cert as file, and we inherit from that
limitation).
Helps making "agent" software release more independent from agent parameter
needs.

Also, avoid single-use local variables.

Reindent and remove "%" nesting.

- rootdirectory section is implicitely pulled by instance section, so do
  not name it in parts.
- merge rootdirectory entries, as mkdirectory created intermediate
  directories implicitly and those entries were not used

Those should not be pushed in public repositories, and should be kept as
local changes/commits on developer machine.

This reverts commit 35c5c950.

It allows to build libcairo.

Decouple the seleniumrunner from the browser and the xserver.

Provides the xserver and the firefox hook.

seleniumrunner code (inside the recipe) still has to be merged with its
original code to prevent duplication.

Allows to run firefox with a new profile each time to prevent having session
conflict.

It allows to start a X server inside a slapos instance.
Provide an hook to do a screenshot of the X session.