Introduce state.IAASModel

[mjs]

Description of change

This is the first of a series of changes that moves IAAS specific functionality from state.State onto a new state.IAASModel type. This is in preparation for new model types. The idea is that State will only deal with generic functionality with model type specific functionality being available via new types accessible via State.

This change moves everything to do with storage onto a new IAASModel type. Future PRs will move more areas of functionality across.

QA steps

All unit tests pass. Deploying a simple storage charm works as expected:

$ juju deploy cs:~axwalk/trusty/storagetest-1 --storage filesystem=tmpfs --storage block=loop
$ juju ssh 0 -- mount  | grep /srv
filesystem-0-0 on /srv/filesystem/1 type tmpfs (rw,size=1024m)

Documentation changes

N.A. - internal only.

Bug reference

N.A.

[mjs]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[rogpeppe]

I'd like to see a bit of discussion in the doc comments about exactly what this "IAASModel" is and how it differs from any other Juju model. "Infrastructure as a service" is an very abstract thing - I have no idea from looking at this PR, or glancing through the code what this change is for or what the name actually implies.