RAPI: Fix resource for replacing disks

Commit d1c172de inadvertently changes the
“/2/instances/[instance_name]/replace-disks” resource to use body
parameters. There were no QA tests and the issue wasn't noticed.

This patch re-introduces support for query parameters and adds a QA
test.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Andrea Spadaccini <spadaccio@google.com>