overlord: allow upload API to check permission

Add GET handler to /api/agent/upload/{mid} to perform permission check
before uploading file. This allow the UI to display error if there is
one.

BUG=chromium:517520
TEST=test with ovl command in CL:291940

Change-Id: Icecaf60657a22e239da0fef0874122ebea302f79
Reviewed-on: https://chromium-review.googlesource.com/303313
Commit-Ready: Wei-Ning Huang <aitjcize@gmail.com>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Rong Chang <rongchang@chromium.org>
4 files changed