Update gitian build to use debian stretch

Summary: Xenial ship outdated version of mingw that cannot compile Qt. Canonical completely fubared LXC support for bionic, so the most sensible alternative path is to move to debian and use stretch. Hopefully we'll get more stability there.

Test Plan: Run the gitan builds for linux, windows and osx.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1840