environs/config: fix optional values/defaults

environs/config: fix optional values/defaults

We have config values which are optional for
backwards compatibility. In some cases these
should take on a different value to what the
default for a new config is. The way the defaults
map is currently constructed means that the
backwards-compatibility values always overwrite
the defaults.

https://code.launchpad.net/~axwalk/juju-core/config-defaults/+merge/207609

(do not edit description out of merge proposal)

[axw, 2014-02-21 10:10:26]

Please take a look.

[wallyworld, 2014-02-24 02:40:27]

LGTM