[TECH DEBT] Required values should be required

[lkubb]

Description of the tech debt to be addressed, include links and screenshots

Functions in this extension declare default values for all parameters, even if they are required. This results in a suboptimal interface and code layout (which needs to check for non-falsy required values).

Example:

message is a required parameter for pushover.post_message. It's the third positional argument though, after user and device, both of which are read from the configuration if unspecified. Thus one cannot simply call salt["pushover.notify"]("My message").