Add caching lsb_release parser

Add caching lsb_release parser

In far too many places do we remotely exec lsb_release in some form,
then parse the output. In shifting to a more stable interface between
teuthology and its tasks, this seemed important to me.

Remote objects now have a 'distro' property that, when first accessed,
calls 'lsb_release -a' remotely and parses the results.

I've included tests and documentation.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>