Move ownership of libweave config to DeviceManager

It was not pretty that global config was owner by class responsible for
cloud communications.

Change-Id: I1476c610662a9773d38eb2fc64e1e2a5c769b63c
Reviewed-on: https://weave-review.googlesource.com/1826
Reviewed-by: Alex Vakulenko <avakulenko@google.com>