utils/ssh updates

[Martin Packman]

I've poked utils/ssh a bit during gophercon.

Unfortuately the update to use tip of go.crypto can't land right now
as it requires a couple of standard library crypto bits that go 1.1.2
lacks. Perhaps we can talk about bumping the requirement for building
trunk juju-core to go 1.2 and adding that in our golang ppa.

The branch to not generate lots of rsa keys for the tests did land,
which makes the testrun a little faster... or a lot faster on arm.

Before:
<http://paste.ubuntu.com/7338971/>
ok  	launchpad.net/juju-core/utils/ssh	375.048s

After:
<http://paste.ubuntu.com/7338978/>
ok  	launchpad.net/juju-core/utils/ssh	4.533s


My chromebook is now much happier... except when it has to run lbox.

Martin