Use a constant for user.toml
Use a constant for user.toml

It's used in several places, so a constant makes sense - it will be
helpful in avoiding possible typos in strings that wouldn't be caught
by a compiler.

Signed-off-by: Lorenzo Manacorda <86995a091f8a1385082bd10715048b52ee386a6a@kinvolk.io>

habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat