Major change is that slapos compatible wrapper is no longer installed in
parts/firefox/firefox-slapos , but directly as "firefox" in the
buildout:bin-directory of the software profile.
geckodriver is also in the same buildout:bin-directory.
This way, softwares using this just need to add
buildout:bin-directory to $PATH

Adjust seleniumrunner and jstestnode for the changes

Also drop unused firefox 45

Jérome changes from original 8cf6908df752b9e7632b2910ffc85027f1f4f22c :
 - name section [firefox-install], because including the version in
section name means we have to also update section name when we update
version.
 - use same version, the "versions up" will be separate commits.
 - now that seleniumrunner and jstestnode are using buildout.hash.cfg,
md5sums are in separate files.
Co-authored-by: Jérome Perrin <jerome@nexedi.com>