init function should validate user input

It is currently possible to run pmbootstrap init and enter arbitrary values into the prompts. This function should validate that the user's input is a valid input for the given prompt, based on a set of acceptable values. Perhaps pmb.helpers.cli.ask can be extended to also include passing a set of valid input values, and in the case of an invalid value entered by user it can provide an error, reprint acceptable values, and re-prompt until the user gets it right.