Properly handle backend add.

* validate backend before adding,
* warn with proper msg if failed backend already added
Fix add_backend user handling.

We need to make sure that the user dict is updated before validating.
The same stands for removing the backend after checking that libcloud
cannot connect.
Properly handle backend add.

- validate backend before adding,
- warn with proper msg if failed backend already added

We need to make sure that the user dict is updated before validating.
The same stands for removing the backend after checking that libcloud
cannot connect.
Automated build of mist.js
fix backend button positioning
Merge branch 'staging'
Merge branch 'master' into baremetal
Automated build of timestamped mist.js
Merge branch 'master' into baremetal