Fix handling of deprecated tools-url

Fix handling of deprecated tools-url

The branch ensures that regardless of which combination
of tools-url and tool-metadata-url values are in the
environment.yaml, the constructed config struct has
both tools-url and tools-metadata-url attributes, both
of which are set to the first non-empty value of, in
order of precedence:
1. tools-metadata-url
2. tools-url

This ensures the config passed to older nodes during
an upgrade still contains the older tools-url that those
nodes rely on.

https://code.launchpad.net/~wallyworld/juju-core/upgrade-fix-tools-url/+merge/195201

(do not edit description out of merge proposal)

[wallyworld, 2013-11-14 11:05:25]

Please take a look.

[jameinel, 2013-11-14 11:11:05]

LGTM