add an optional attribute called requires-roaming to the provider

Add an optional attribute called requires-roaming to the provider.  By
default this is false, but it is set to true for Cubic.

BUG=chromium-os:5370
TEST=xmllint --valid --noout serviceproviders.xml
TEST=run unit tests

Change-Id: I4c0c1d6cc39a499a0baf164d7cd31d9632495f31
6 files changed