Migrate precheck allow hook execution

[howbazaar]

Loosen up the precheck criteria to allow for hooks to be executing.

This is very important due to the workload status hook that executes every thirty seconds. Given a model of large enough size, it is reasonable to consider that there would always be some hook executing somewhere.

We do need to consider perhaps an extra precheck at a later stage to ensure that no agents are in an error state.

[mjs]

Oarsum

[howbazaar]

$$merge$$

[jujubot]

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