cmd/juju: juju takes 3 seconds to do nothing

[Dave Cheney]

Testing juju on a platform that does not have a fast assembly implementation of the math/big package leads to weird 3 second delays even when juju is doing nothing

$ time juju hey dave
ERROR unrecognized command: juju hey

real 0m3.410s
user 0m3.394s
sys 0m0.013s

It turns out that during startup, juju is reading and decoding its private key

close(4) = 0
open("/home/ubuntu/.juju/ssh/juju_id_rsa", O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0600, st_size=1679, ...}) = 0
read(4, "-----BEGIN RSA PRIVATE KEY-----\n"..., 2191) = 1679
read(4, "", 512) = 0
close(4)

This is wasteful even on platforms that have a fast crypto library.