Allow users to add private clouds

It would be nice to allow for users to add their own private clouds, and optionally publish/share them, after being approved.

This is mostly useful for OpenStack clouds, but could also be useful for EC2 compatible clouds, and in future, Azure Stack based clouds.

Tasks
- [ ] Extend backend to store private cloud connection information.
- [ ] Add front-end component to define a cloud connection. Could be called My Connections, available through the My Profile page.