Generate "system identity" in jujud bootstrap

I recently moved the system-identity generation
up the stack from each of the providers into
environs/bootstrap common code. This caused a
slow down in tests that weren't previously
generating the key as they use the dummy provider.

This should never have been in the client side;
I'm moving generation of the system identity
into bootstrap.

Fixes https://bugs.launchpad.net/juju-core/+bug/1361971
Should we check the contents as well?