variable/conditional arguments for an action

aws actions can have multiple optional arguments which affect the behaviour. When building a workflow there'll be occasions when it'd be useful to only input the information i currently have as an input

example:

aws rds --region eu-west-1 describe-db-engine-versions --engine postgres
aws rds --region eu-west-1 describe-db-engine-versions --engine postgres --default-only
aws rds --region eu-west-1 describe-db-engine-versions --engine postgres --engine-version 9.6 --default-only

each of these commands (or the st2 equivalents) will give a different response. Where there's only 1-2 argument differences this isn't too big a deal, but for things like aws_rds.create_db_instance there are many different flags that can be optional, so it becomes less feasible to have multiple versions of the same action with flag combinations

It'd be useful to be able to have conditionals on inputs, so if a var is set then the input is set

No files, tests, or entry points are named. Start by locating StackStorm's action input and workflow argument handling, then compare it with the AWS examples; done means optional inputs are passed only when their variables are set, without requiring separate actions for each flag combination.