This enables deb-src repos on Debian hosts with a flag. The flag
currently defaults to True to maintain backward compatible behavior with
the hardcoded deb-src repos that configure-mirrors previously set. We
intend on flipping this default to False after announcing this change.
The reasons for this are that source package repos are rarely needed
by CI systems, but these repos can consume large amounts of disk in
mirrors. To make it easier to people deploying mirrors we want to avoid
enabling source repos by default.
Change-Id: I7e9cd0ec1e3184c3c0561bbb7d3069feaf5f1ba5
fc133bd165