builder: Add --print-cache option.
builder: Make --delete-cache obey the --cache option.

Previously a command such as:

  virt-builder --cache /tmp/cache --delete-cache

would delete ~/.cache/virt-builder (the default cachedir).

It's obviously meant to delete /tmp/cache, which this commit fixes.
builder: Add --cache-all-templates option.
builder: Advice on setting environment variables to maximize caching.