Add support for Lambda Labs

[gmittal]

This is a (very rough) first pass at implementing a node provider for Lambda Labs which has some of the cheapest cloud GPUs available.

TODO

• [x] Single-node support (e.g. sky gpunode works e2e with failover)
• [ ] sky check setup
• [ ] Clean up clusters correctly (remove SSH keys, etc.)
• [ ] Implement multi-node
• [ ] Lots of tests... (YAML, partial cluster provisioning failure, etc.)

All suggestions and feedback welcome!