cluster join: provide token as mandatory argument

[James Page]

For a join operation the token is mandatory so should not be provided as a command line option.

Current:

  sunbeam cluster join --role control --role compute --role storage \
     --token YmRlODViYjYtMGFlNy00MmFjLWE4NzMtNjI0ODg4YmUzZTM0Cg==

Proposed:

  sunbeam cluster join --role control --role compute --role storage \
     YmRlODViYjYtMGFlNy00MmFjLWE4NzMtNjI0ODg4YmUzZTM0Cg==

Lets deprecate --token and use an argument instead or read from STDIN to prevent it appearing in the process listing for the machine.