New config option 'webroot', config-changed and upgrade support.

Adds a new config option 'webroot' that can be used to set the path
at which dashboard is accessible.

Adds a new config-changed hook that allows this option to be reset,
and also allows Essex -> Folsom upgrading.

Adds support for using cloud:precise-folsom/staging as an installation source.

config.yaml

@@ -7,3 +7,9 @@ options:
           Valid choices are a LP PPA ('ppa:your/ppa') or a URL entry for
           sources.list with an optional GPG key appended after | for import
           from Ubuntu keyserver ('http://host/ubuntu oneiric main|KEYID')
+    webroot:
+        default: "/horizon"
+        type: string
+        description: |
+            Directory where application will be accessible, relative to
+            http://$hostname/.