Allow providing default provider with Nulecule file.

[rtnpro]

This allows specifying a defaultprovider for a Nulecule app in the spec file in the metadata section:

metadata:
  name: Hello Apache App
  appversion: 0.0.1
  description: Atomic app for deploying a really basic Apache HTTP server
  defaultprovider: docker
...

In case you don't have provider specified in the [general] section of your answers.conf file, we first look for metadata.defaultprovider in Nulecule spec file. If no default provider is specified in
Nulecule metadata, we'll default to lib.constants.DEFAULT_PROVIDER for the provider, which is, kubernetes.

Fixes #378

[cdrage]

Looks like a test failed :(

Otherwise, it looks good to me.

What's the format again @rtnpro ? We add "defaultprovider" to the meta-data right? We may need a doc for this somewhere. Do we need to update the Nulecule spec too?

[rtnpro]

Obsoleted by #522