More flexible application_name

### Tell us about your request
It would be very useful if it was possible to set `application_name` as a flag of `eb deploy`.
e.g. `eb deploy --application-name my-app-staging-us-east-1`

### Tell us about the problem you're trying to solve.
We sometimes want to deploy a given branch to more than one application in a region e.g. `my-app-staging-us-east-1` and `my-app-funfunfun-us-east-1`. Currently we have to edit `config.yml` every time, which doesn't lead itself well to scripting.

### Are you currently working around this issue?
Yes. I wrote a shell script which takes environment as a parameter, generates `config.yml`, and runs `eb deploy`.

Start at the `eb deploy` command and inspect how `config.yml` supplies the application name. Trace the command's argument parsing and configuration lookup, then verify that `--application-name` can select a target application without editing `config.yml`, including the scripting use case described.

Written by the indexing model from the issue text.