juju bootstrap fails if kvm-ok not in path

[Adam Stokes]

We've run into an issue where if the users PATH does not include /usr/sbin then kvm-ok will fail during a juju bootstrap attempting to use container type KVM. This causes the entire bootstrap process to stop and leave users with an uninitialized juju environment.

Most default Ubuntu installs contain /usr/sbin in /etc/environment but some cases like this one where this may be from a cloud provider using a stripped down version of Ubuntu as part of their builds will cause juju bootstrap to fail.