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
I suppose this is an unlikely case, which you could deal with using YAML if you needed to. The error