Move parse search logic outside from builder

Dramatic part of RingBuilder.search_devs which parse a complex format
of a search device string moved to the swift-ring-builder script.
Instead, the search_devs has a simple interface to search devices.

blueprint argparse-in-swift-ring-builder

Change-Id: If3dd77b297b474fb9a058e4693fef2dfb11fca3d