Toggle modules per environment.

[danepowell]

Fixes #668 .

Changes proposed:

• Modules will be toggled based on the value of an "environment" parameter.
• No functional change for the ci:setup, ci:update, local:setup, and local:update targets (we are simply consolidating variables)
• For deploy:update, you will now need to define modules to toggle per environment. But if you prefer the old style, simply don't pass the environment parameter and it will default to deploy like before.

[danepowell]

Note that at some point, we could take this even further and have an "environment" array that defines more than just modules per-environment. For example:

environment:
  local:
    modules:
      enable:
        - devel
      uninstall:
        - acquia_connector
    drush-alias: self
    uri: my-project.local // Use to set trusted_host pattern, base_url, etc
    caching: disabled // Use to enable/disable caching in a settings file
  dev:
    ...

We probably don't want to do all of that as part of this PR, but maybe we want to at least rename the array from "modules" to "environments"?

[grasmash]

How about setting environment to dev in project.local.yml?

[danepowell]

You could do that, it just seems like local is a distinct environment. For instance, you don't want shield locally, but you do want it on dev. And I think there's nice symmetry between the environments and ci:* and local:* wrappers.

At any rate, these are just suggested environments, projects can define whatever environments they want.