Compile YAML only once

[eliaskioni]

Airflow compiles the ussd screens in the yaml file for each new request. We can improve on this.

On average on aws t2 medium instance it takes 234 ms to compile this file.

With an aim to respond to MNO's requests at most within 2 seconds, its urgent we rethink how to compile the yaml screens file once during start up.

234 m/s is fine until you start hitting a high number of requests.

Any suggestions on how to well handle this will be appreciated.