Ensure step field values are always strings
Ensure step field values are always strings

Env vars must be strings, and most step field types are strings, but
"boolean" field types come through as True / False.

Fixes #1261

ubuntu/conjure-up,conjure-up/conjure-up,ubuntu/conjure-up,Ubuntu-Solutions-Engineering/conjure,conjure-up/conjure-up,Ubuntu-Solutions-Engineering/conjure