This new option for slapos.cfg allow to define a list of paths to use
for shared parts feature of `slapos.recipe.cmmi`.
This will be injected in the buildout profile used to install software,
as `${buildout:shared-part-list}` which is the option `slapos.recipe.cmmi`
use since 0.11.

Note that the option changed in `slapos.recipe.cmmi` because this was
incompatible. In 0.10, the option was named `${buildout:shared-parts}` and
it was supposed to be a single path. `slapos.recipe.cmmi` 0.10 was trying
to create this path. If we continued using this `${buildout:shared-parts}`
option and changed to passe a newline-separated list of paths, then
`slapos.recipe.cmmi` would just create one directory with \n in the name
and will probably fail, which would make all software from 1.0.73 to 1.0.121
not installable on next `slapos.core`, so we used another buildout option
to prevent this problem.

Because generally, profiles using slapos.recipe.cmmi 0.10 were not
"safely" shared, we don't even provide compatibility with old option.

/reviewed-on nexedi/slapos.core!46