Add the concept of a "default" repository

[dakrone]

I think we should introduce the concept of a "default" repository. This is a snapshot repository that could be added but not totally removed, and we could then make use of this as a default when specifying any action that takes a repository (SLM policy, Searchable Snapshot ILM action, etc).

Forcing a user to confront the need to configure a default repository sooner rather than later will be helpful for users to get the full benefits of our integration with things you can do with a repository. This user confrontation could be in the form of warnings on ES startup, default alerts within Kibana, or outright refusal to start without a configuration (that one might be a bit much), but in general, anything that leads a user to configure a default snapshot repository sooner rather than later.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)