align script and image versions
- released weave scripts run their corresponding image versions
- unreleased weave scripts run the 'latest'-tagged image versions

The image version to run can also be specified explicitly by setting
VERSION. Useful for testing, mainly.
don't include image version when checking for existing containers

So we complain with "already running" rather than "Found another
running container" when encountering a 'weave' container running a
different image version. Similarly, we remove stopped 'weave'
containers regardless of version.

Ditto for weavedns.

Which is all how it used to work.
Merge branch '34_version_alignment' of https://github.com/rade/weave into pr306_lock_versions
Merge branch 'pr306_lock_versions'