[k8s] Parallelize multi-node setup

[romilbhardwaj]

Supersedes #4261 and #4270.

We identified that apt update is the slowest operation and triggering it after setup takes a lot of time, so we now inline it in container init args to let kubernetes run it in parallel.

Testing on nemo image

sky launch -y -c test --num-nodes 100 --cloud kubernetes --image-id nvcr.io/nvidia/nemo:24.05.01

Master branch: 19:56.21 total

This branch: 15:26.56 total

Testing on default image:

sky launch -y -c test --num-nodes 100 --cloud kubernetes

This branch: 4:13.41 total

Tested (run the relevant ones):

• [x] Code formatting: bash format.sh
• [x] Manual testing with 100 nodes on nemo image, our base image and miniconda image.
• [ ] Need to run relevant smoke tests.