[RFE] Support Storage Resizing

[Pedro Guimarães]

There are two aspects of this feature request:

1) short-term - how to reset storage profile of a given application after a manual resize happened

   This is a more immediate change: how one can manually resize one of Juju's storage-managed disks and then set Juju so the next units of that application come up with the newer size value
   With machine constraints, that is possible by using "set-constraints" to reset "root-disk", for example.
   Juju storage APIs need something similar for its storage constraints

2) long-term - to provide APIs and events that allow to resize storage disregard of the underlying cloud.

   In this case, juju support a set of APIs that allow to resize a given volume.
   It should also provide events to inform the application that a resize is going to happen.

   For example:
   resize-requested: informs the unit which disk will be resized
   resize-done: informs the unit the resize operation took place and successfully finished.

Ideally, the application should have a say in the resize event.

[John A Meinel (jameinel)]

So the plan is to support part (1) with the same mechanism that we have for other constraints. To allow ways to ask juju to update storage constraints (like we can `juju set-constraints` for changing mem/cpu) and have those used for any additional units (during scale up/down).

This is likely to be expressed as `juju storage-constraints size=11GB`.

This doesn't give an API to trigger the actual reallocation, but it does allow updating the "when starting a new one, make sure these attributes apply".