attestation: add a command line switch to specify the VA servers

One can use --va_server=default|test to specify the VA servers to use
when signing enterprise challenges.

BUG=chromium:793122
TEST=manual

Change-Id: I6d3651011e944621f44786505b3ff93fd9b836d3
Reviewed-on: https://chromium-review.googlesource.com/818633
Commit-Ready: Yves Arrouye <drcrash@chromium.org>
Tested-by: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Andrey Pronin <apronin@chromium.org>
1 file changed