fix(archlinux): avoid rendering error

[noelmcloughlin]

Use debian:stretch as default for all OS (override in osfamily map).

Fixes this on Archlinux

SaltRenderError: Jinja variable 'dict object' has no attribute 'platform'
[CRITICAL] Rendering SLS 'base:opensds.backend.block.config.cinderbox' failed: Jinja variable 'dict object' has no attribute 'platform'
local:
    Data failed to compile:
----------
    Rendering SLS 'base:opensds.backend.block.config.cinderbox' failed: Jinja variable 'dict object' has no attribute 'platform'
Salt Version:
           Salt: 2019.2.0

[noelmcloughlin]

Resolves #105