Use password from .jenv files for juju api access

For non-local installs, the juju api password is not the same as the openstack password.
This just uses the password from the .jenv in both local and non-local.

It also caches the .jenv data in the config obj to avoid re-reading it all the time, which seems OK to me but without knowing how juju handles those files, it could be a source of subtle bugs and may not be worth the tiny runtime savings.

Opinions?