Automatically update weave + weavedns images

[yaronr]

After a short discussion with squaremo:
It appears that weave does not take care of updating its docker images.
My point of view is:
Either getting the images (and their existence, for that matter) is the user's responsibility, or not.
If it's the user's responsibility, then the user should take care of updating the images.
Otherwise, if this is abstracted from the user - then it becomes weave's responsibility.

[rade]

Always checking/grabbing the latest version a) can break things (if the latest version is not compatible with the running script), and b) would significantly delay things (since the docker hub is slow as snails, and you need an internet connection).

Following #34, when the weave script finds that the local docker images are too old, it could attempt to fetch the most recent compatible versions.

[hesco]

Upgrades ought to be the call of a human sysadmin.

That choice ought to be facilitated

• by semantic versioning (Issue semantic versioning #110 and PR resolve #110, provide for semantic versioning  #165),

• tools for introspection (weave version and weave status)

• automated version checks to advise humans of upgrade opportunities like what is proposed at:

  ** new class needed to provide version check
  ** extend weave::install to support ensure => latest,