[ui] Improve the process to align application to service env vars

[richard-cox]

• When a user has an application it may have env vars that require values from a bound service, for instance the epinio/example-wordress app requires DB_HOST (epinio configurations list | grep mydb | awk '{print $2}')
• Currently in the UI the user has to create the service in advance, create the app (sans service env vars) bound to the service, find the new configuration, and then edit the app
• This isn't intuitive or provide a good UX

Suggestion

• We should work out a way for the user to see the env vars the configuration exposes and assign value/s to application env vars
• This is tricky because it requires both app and service to exist and service bound to the app

[richard-cox]

Before the UI starts this we need to talk as a team the best way to resolve it.