add role reader-> AERO_ROLE env var

[teaforthecat]

I didn't see another way to do this in aero, so I thought I would add it here. The idea is to use the same config for multiple deployments, of which an example is on the way.

[gws]

I'm hoping an example will help me understand the problem this is solving, but you might put that in the description of the PR - there might be a different way to accomplish what you're trying to do.

[teaforthecat]

So with https://github.com/democracyworks/electorate-works/pull/93, as the example, does this make sense now?

The idea is the profile/environment case-like statement can't be used for both the environment and this purpose. There may be a way to do it with a ^:ref and #env but I thought this was cleaner. If you can see a better way please do share.

[gws]

What I took away from https://github.com/democracyworks/electorate-works/pull/93 is that you might be trying to do too much with a single configuration file. Mike suggested a second configuration file and I suggested configuring the "solo job" queues as a sibling key in the existing configuration. I would prefer either of those (for a bonus, also switching away from this library) over the conditional logic based on AERO_ROLE.

As for the naming, I'm not sure we should use AERO_ROLE because it's not something that will be added to aero and so doesn't belong in that namespace.

Finally, the plan is to decommission this library altogether, so locking services in to this library because of bespoke functionality isn't ideal. If aero doesn't suit your needs for configuration there are other good libraries out there that don't have quite as many warts as this one, but ultimately I don't think a configuration library makes sense for Democracy Works to maintain.