DOCKER-467: create config settings for server and api version

[twhiteman]

Summary of changes:

• new etc/defaults.json file
  • not sure why another config file - why not use the sapi manifest file?
• docker server API version (1.19) and docker server version (1.7) moved to etc/defaults.json file
• sdc-docker app config first loads the default config (etc/defaults.json), then loads the sapi config (etc/config.json) and then merges these two config settings (sapi wins over default) to create the app.config
• update various places/tests where server/api version is used

[trentm]

@twhiteman is etc/defaults.json in this PR?