Update docstrings in tools/ and enable epydoc

This patch updates the docstrings in tools/ (mostly in lvmstrap, which
is very very old code-base) and then enabled the tools in this directory
for 'make apidoc' too.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>