Allow deploying to an existing host

Allow a user to specify a username and host address and deploy an application to it. CloudLaunch will have a special private/public SSH key pair and will instruct the user to add the public key to host.

This requires a few different changes:
* [ ] separate out logic of configuring a host from the app plugins so that it can be reused
* [ ] change the data model so that deployments can be associated with a more general "deployment target" than Cloud
* [ ] some UI changes to allow a user to specify a host instead of picking a cloud