Revert "software/erp5: remove software-shared.cfg"

This reverts commit ef5aca54.

software-shared.cfg is used on master from scalability tests, let's keep
it for now.

software/erp5/software-shared.cfg

+[buildout]
+extends = software.cfg
+shared-parts = /opt/slapgrid/shared-parts
+eggs-directory = /opt/slapgrid/shared-eggs
+abi-tag-eggs = true