Create a script which creates a global make.conf file which is
read by poudriere when building all ports.

Set WANT_OPENLDAP_SASL in this file.
Remove WANT_OPENLDAP_SASL from os-ports.  It is a global option which belongs
in the make.conf file read by poudriere when building ports.
Move logic for parsing SW_FEATURES out of os-base and into nano_env.
This lets us set:
  SW_FEATURES="REPORTING"

which will in turn set:

  REPORTING=1

This setting will be valid for all scripts which include nano_env.
Without this, we were building an image with poudriere, with none
of the ports required for the reporting module.