Optimizations in host file generation scripts

[adityaku-ms]

Made the following changes:

• Optimization to call the app server list API to fetch IP details of active app servers instead of relying on pinging each hostname separately. In case this API call fails, fall back to pinging each hostname.
• Added de-duplication in the bash script to handle cases where there may be multiple hostnames in the same VM.
• Added handling for various error scenarios.
• Updated the README accordingly.