component/proftpd: use slapos.cookbook:userinfo

slapos node software fail with:

```
We have no distributions for lxml that satisfies 'lxml==3.7.3'.
Building lxml version 3.7.3.
Building without Cython.
ERROR: /bin/sh: 1: xslt-config: not found

** make sure the development packages of libxml2 and libxslt are installed **

Using build configuration of libxslt
Compile failed: command 'gcc' failed with exit status 1
*********************************************************************************
Could not find function xmlCheckVersion in library libxml2. Is libxml2 installed?
*********************************************************************************
An error occurred when trying to install lxml 3.7.3. Look above this message for any errors that were output by easy_install.
While:
  Installing.
  Getting section instance-profile.
  Initializing section instance-profile.
  Getting section proftpd-output.
  Initializing section proftpd-output.
  Getting option proftpd-output:command.
  Getting option proftpd-output:proftpd.
  Getting section proftpd.
  Initializing section proftpd.
  Getting option proftpd:environment.
  Getting section proftpd-userinfo.
  Initializing section proftpd-userinfo.
  Installing recipe slapos.cookbook.
Error: Couldn't install: lxml 3.7.3
```

component/proftpd/buildout.cfg

@@ -4,12 +4,8 @@ extends =
   ../openssl/buildout.cfg
   ../zlib/buildout.cfg
 
-[proftpd-environment]
-recipe = collective.recipe.environment
-# This component depends on https://nexedi.erp5.net/bug_module/20180402-716ACB/ being fixed
-
-[proftpd-grp]
-recipe = collective.recipe.grp
+[proftpd-userinfo]
+recipe = slapos.cookbook:userinfo
 
 [proftpd]
 recipe = slapos.recipe.cmmi
@@ -24,8 +20,8 @@ configure-options =
 environment = 
   CPPFLAGS=-I${zlib:location}/include -I${openssl:location}/include
   LDFLAGS=-L${zlib:location}/lib -Wl,-rpath=${zlib:location}/lib -L${openssl:location}/lib -Wl,-rpath=${openssl:location}/lib
-  install_user=${proftpd-environment:USER}
-  install_group=${proftpd-grp:GROUP}
+  install_user=${proftpd-userinfo:pw-name}
+  install_group=${proftpd-userinfo:gr-name}
 
 
 [ftpasswd]