cmd/juju: permit user@ in scp/ssh target

The "juju ssh" and "juju scp" commands now
allow "user@" in the target. For example,
you can now do "juju ssh jenkins@jenkins/0"
to ssh to the first jenkins/0 unit as the
jenkins user.

Also, fix examples and clarify help for
"juju scp" regarding passing additional
arguments to "scp".

Fixes https://bugs.launchpad.net/juju-core/+bug/1387766
Fixes https://bugs.launchpad.net/juju-core/+bug/1387640