Hiding such environment variable prevent easy access to the user
owning the software or the user owning the instance.

Some instances needs to have the actual username of the instance in their
config files and some softwares needs access to the username of user
building the software. This is - in theory - easy to do in zc.buildout
thanks to https://collective.github.io/collective.recipe.grp/ , but the
way slapos run buildout with some environment variables hidden made it
not possible.

Fixes [#20180402-716ACB](https://nexedi.erp5.net/bug_module/20180402-716ACB/)

/reviewed-on !41

We only had a global "pidfile" configuration, but it's more conveniant
to be able to configure different pidfile for each command (software,
instance, report)

The supported config file would look like:

```
[slapos]
pidfile_software = /path/to/pid
pidfile_instance = /path/to/pid
pidfile_report = /path/to/pid

```



/reviewed-on !29

  This version include initial plugin system and cgroups integration.

  We also included a significant change for provide the partitions network informations, which now
  don't require to come from slapos master anymore, so it is possible to make local extensions.

Also fill missing changelog.

slapos format: fix use_unique_local_address_block feature, and put default to false in configure_local.