Refine index priority order of pip install

[jialeicui]

Current Behavior
Merge all index URLs together and it may cause security issues.
ref: pypa/pip#8606

Proposed Behavior

Index = index-url or extra-index-url

There are 3 places to specify Index

1. requirements.txt
2. runtime.yaml
3. Cluster ( configured in the controller)

Rules

• Index in the front position has the higher priority.
• Prefer using the current Index when installing packages.

e.g. When installing packages in runtime.yaml, use Index in runtime.yaml if configured, else: use Cluster Index

cc @tianweidut