feat: Health check and health command

[mprey]

Added:

• health check via TCP ping
• health command given a VM name
• health status in slim vms

Notes:

• Should we have health status checks disabled for slim vms but enabled via an optional flag?

[ssmirr]

I encountered two issues when running the PR:

1. slim vms only shows one of my vms.
   I think I know the reason for this. The problem is in using async/await inside reduce(). This blog post has a good explanation for what the issue is and how to fix it. I also had another comment about this above ^ (in the code review).

2. the tcpPing(sshInfo) result is an object like below, so !!result will be true even when it is failing to ping:

   {
     address: '127.0.0.1',
     port: 80,
     attempts: 5,
     avg: NaN,
     max: undefined,
     min: undefined,
     results: [
       { seq: 0, time: undefined, err: [Error] },
       { seq: 1, time: undefined, err: [Error] },
       { seq: 2, time: undefined, err: [Error] },
       { seq: 3, time: undefined, err: [Error] },
       { seq: 4, time: undefined, err: [Error] }
     ]
   }

I think it's reasonable to make health check run with an optional flag for slim vms.

[mprey]

Fixed the two issues in most recent commit

[ssmirr]

Thanks for the PR and great work @mprey! LGTM.

[ssmirr]

I thought doing ping for all the vms would be slow, but since it is pretty fast, we changed --health to run as default. 37a89e1