grid: new shared_part_list config file option
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.