This adds the _update_state() method to the Element class to keep track
of element state and avoid calculating the same cache key multiple
times. This also consolidates the different get_cache_key
methods into a single method that always returns the cache key
calculated by _update_state(), if available.

Fixes #149, #173

The initial multiple cache support patch implemented a rather fragile
logic where we would push to the first cache in the list that used the
ssh:// protocol, if any. If we implement push-over-https:// in future
then this will become totally unworkable.

This patch alters the logic so that each remote has a 'push' option,
and BuildStream will push to any remote that has 'push: true' set.

This allows pushing and pulling from a specific cache, ignoring what is
configured.

If we choose to add a --remote option to `bst build` in future that
would now be simple to do.

This extends the 'artifacts' configuration block such that a list of
`url` mappings can be given instead of a single entry. For example:

    artifacts:
      - url: http://example.com/artifacts1
      - url: ssh://ostree@example.com/artifacts2

The OSTreeCache class is updated to set up multiple remotes and query
remote refs from all of them.

There are no automated tests for this yet.

Empty URLs ('') now raise an exception. They cause breakages internally
if we allow them through, and they can only occur if the user or our
tests are misconfiguring things somehow.

We report failure to fetch from the cache by printing a message to
stderr for now. This is because BuildStream's actual logging
functionality can't be used during frontend init -- see issue #168.

Dont make the ERROR message explicitly anymore, leave that
to the frontend to sort out.

If a plugin raises an error, prepend the plugin identifier to the
error message and raise PipelineError.

In addition, it changes the "plugins" and "required-versions" fields,
combining them for plugins and adding a new "required-project-version"
field.