Add CONF_METAPKGS to select cart metapackage deps

Added new configuration variable, CONF_METAPKGS, which allows users to
selectively install `optional`, `recommended` or `none` cartridge
dependency metapackages.

Default is `recommended`

`parse_cartridges` is updated to only install the cartridge metapackages
for the selected cartridges

Squashed commit:

Rewrite parse_cartridges metapkg logic

Updated `parse_cartridges` metapkg logic to rely on the resolved value
of `pkgs`

Rewrote `metapkgs_optional` and `metapkgs_recommended` to not use `grep`
Merge pull request #310 from dobbymoodge/conf_metapkgs

Add CONF_METAPKGS to select cart metapackage deps