PNDA-4055: Feature flags in YAML config not easily extensible

[piyushpote1]

Problem Statement:

Dependency PR: pndaproject/platform-salt#579
PNDA-4055: Feature flags in YAML config not easily extensible

Changes:

• Updated pnda_env_example.yaml. Configured features as list of features instead of key-value pairs.
• Renamed EXPERIMENTAL to a specific optional feature name i.e SPARKMAGIC
• Updated backend_base.py to handle the list in the yaml file.

Test details

RHEL(CDH pico and HDP pico)

[trsmith2]

This is a good improvement but the specific features of livy/sparkmagic are no longer supported. Be great to get the general improvement into the code if you can remove the specifics that have since been deprecated.