azure: Move to vs2017-win2016 platform build host

Azure is moving to remove the vs2015-win2012r2 platform build host.  The
two suggested new platforms to use are vs2017-win2016 and windows-2019.
For now, move up to vs2017-win2016.

Cc: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>