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

erp5testnode knows since
erp5@59ee7970
that it should look for firefox in buildout:bin-directory of the
installed SR, so we should not need backward compatibility here.

Adjust seleniumrunner and jstestnode for the changes

Also drop unused firefox 45

Jérome changes from original 8cf6908d :
 - name sections [firefox-wrapper] and [firefox] because including the
version in section name means we have to also update section name when
we update version. Users have to be careful of installing
${firefox-wrapper:} and not ${firefox:}
 - 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>