The original code would fail on pass paths with spaces,
because they would be split into multiple arguments.

Just set $HOME for calling the pass binary
to avoid affecting anything outside the pass renderer.

1. Allow us to make the pass renderer fail during pillar rendering
   when a secret corresponding with a pass path cannot be fetched.
   For this we add a master config variable pass_strict_fetch.

2. Allow to have prefix for variables that should be processed
   with the pass renderer.
   For this we add a master config variable pass_variable_prefix.

3. Allow us to configure pass' GNUPGHOME and PASSWORD_STORE_DIR
   environmental variables.
   For this we add master config variables pass_gnupghome and pass_dir.