cbuildbot: archive the manifest we used
cbuildbot: archive the manifest we used

Since the exact manifest a bot used to build is useful to developers,
add a step to do that in the ArchiveStage.  The file is tiny so it
shouldn't really impact things.

BUG=None
TEST=Set the snapshot_manifest parameter and make
     sure that buildbot makes manifest snapshots

Change-Id: Ib0a440fcac25a74aed74742b6a20a8a71b34595c
Reviewed-on: https://chromium-review.googlesource.com/49825
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite