Add @ syntax to set command

Added the @ syntax to the set command for setting service options. It allows that the value for an option can be read out of a file:

juju set myservice myoption=@valuefile.txt

See also

https://bugs.launchpad.net/juju-core/+bug/1216967
In general, do we allow setting arbitrary parameter values, or is there some validation (or assumed sanitization on inputs from YAML & args)? Should we restrict to valid UTF-8, and a reasonable size limit, or leave it wide open?