Fix problem with "Text can not be null" when publishing to Forge

A regression in how forge client credentials were passed caused this
message to pop up every time an attempt was made to publish to the
Forge.

This commit fixes the following in the publishing dialog:
1. The Forge Service URI is picked from the preferences so that it
   can be changed to for instance forge-staging-api.puppetlabs.com
   for testing purposes.
2. That the "Save password in secure storage" is moved to the options
   group.
3. That a new option "Dry Run" is added to the options group.