upload tsdb block to cortex

[roidelapluie]

It would be nice if cortex-tool would gain a block upload feature that enables posting a block to block storage, e.g. after a cortex downtime.

[jtlisi]

💯

We currently deploy Cortex using a custom image with some command line tools (gsutil,boto,etc) that would allow for an operator to manually upload blocks to object storage if they were responding to an incident. If we enabled this in the cortextool it would fill this gap nicely and ideally have support for all of the existing TSDB object storage backends.

However, ideally an implementation should be rather lightweight since the cortextool is starting to become a bit bloated. See #100