Refactor out api command generation to valohai-utils

[magdapoppins]

We want to be able to generate an api command for a given step in a yaml file via valohai-utils.

• move part of the vh exec ... code to valohai-utils (get a api command for a given yaml and step)
• make valohai-cli use the above mentioned

customer: Greensteam

[JuhaKiili]

From customer GreenSteam ticket:

hey, is there a way to parse the valohai.yaml file into the api request example in python?
right now I have the python call payload pasted into the module code but I would like the payload to be dynamically constructed from valohai.yaml