`lib/config/configuration.go` app-name and app-uri references should be renamed to name and uri respectively

[mdwn]

Expected behavior:

The error message output by teleport app start should echo the correct CLI flags on a misconfiguration.

Current behavior:

The error message references the wrong CLI flags.

Bug details:

• Teleport version 11.0.0-alpha2
• Recreation steps
  • After properly configuration a cluster, run teleport app start with --name or --uri missing.
• Debug logs

  ➜  11test ./teleport/teleport app start -c/Users/mike/src/teleport-clusters/11test/11test.yaml --uri=http://localhost:9000 --auth-server=https://11test.teleport:443    
  ERROR: application name (--app-name) and URI (--app-uri) flags are both required to join application proxy to the cluster

[zmb3]

This is a bit trickier:

• teleport start takes --app-name and --app-uri
• teleport app start takes --name and --uri

The code emitting the error message is generic lib/config code that doesn't know how the program was invoked.