cbuildbot_stages.py: clarify logging of metadata creation/uploading
cbuildbot_stages.py: clarify logging of metadata creation/uploading

Prior to this CL, the message "Uploading metadata now" was logged
immediately before the metadata object was *created*, which is long
operation involving many GS queries. This CL clarifies the logging and
logs the different stages of metadata creation/upload separately.

BUG=None
TEST=Unit tests pass. Manual local cbuilbot runs show desired logging
behavior.

Change-Id: I0f7734250d84e8c21c56599d3467c632f9bdd4af
Reviewed-on: https://chromium-review.googlesource.com/187940
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Commit-Queue: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Tested-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

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