Lambdas / Factory API smoothing

[BelSasha]

Main changes:

1. Lambda function factory smoothing (removed args_names, paths_to_code, handler_funciton_name)
2. Enabling creating lambda function with a path to code using LambdaFunction.from_handler_file method.
3. Remove tests that became redundant after the refactor.

[dongreenberg]

This is awesome work!! Just had one note about the delete and delete_from_den methods - we can get rid of delete_from_den because delete_configs is already inherited from resource, and can we use teardown as the method to delete the lambda itself to be consistent with cluster?