grid: do not hide `$USER` when running buildout

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

CHANGES.txt

 Changes
 =======
 
+(unreleased)
+------------
+ * grid: do not hide `$USER` when running buildout
+
 1.4.6 (2018-03-29)
 ------------------
  * grid.promise: use previous promise execution result if the promise is skipped because of periodicity.

slapos/grid/utils.py

@@ -66,7 +66,6 @@ SYSTEM_ENVIRONMENT_REMOVE_LIST = [
   'TEMP',
   'TMP',
   'TMPDIR',
-  'USER',
 ]
 
 LOCALE_ENVIRONMENT_REMOVE_LIST = [