Implement init command

[stgraber]

The goal behind the init command is to avoid having the user directly deal with cluster bootstrap and cluster join.

Instead init should provide an interactive workflow where it asks the user whether to create a new cluster or join an existing one.
When creating a new one, it should ask if the user wants to add additional servers to it and if so, issue join tokens.

Once the initial clustering part handled, it should scan available disks and ask the user which should be added as OSDs.

[stgraber]

Current plan for the flow:

 - Check if initialized (/1.0/cluster)
    - YES: Say already initialized ask if want to add disks
        - NO: Exit
        - YES: Jump to cluster add step
    - NO: Ask whether to create a new MicroCeph cluster
        - NO: Ask for join token
        - YES: Bootstrap new cluster
 - Ask if want to add additional machines to the cluster
 - Show local disks available, option to directly enter path or exit
 - <Loop until exit selected>

[stgraber]

We'll eventually want to add the option to deploy additional services to that, but that will need work on the services API first.