[VVP] Generated completed preload from env files

User can supply an optional directory containing .env files and/or
CSAR VSP which can be used to generate populated preloads in the
requested format.

The nested directories can be used to create sub-environments that
inherit their settings from the parent directories.

Optionally, values can be specified in a defaults.yaml and they will
be used if that value is not defined in the .env file.  This is useful
if the parameter name and value will be the same in all modules.

Issue-ID: VVP-278

Change-Id: Icd9846c63463537793db908be8ce5dba13c4bda3
Signed-off-by: Lovett, Trevor <trevor.lovett@att.com>