Issue #166 Fix.

Made single adhoc interface condition explicit about requirement for a "single" adhoc interface. It had been falling back to a state that assumed at least one ad-hoc interface existed without asserting it.

per: https://github.com/opentechinstitute/luci-commotion/issues/166
@areynold 

To test this pull request. 

* Create a build with this branch of luci-commotion
* Configure node though setup wizard
* Go to the menu Basic Configuration --> Mesh Network 
* Click on the delete button of the mesh interface
* Click save
* Apply the changes
* Go to Security --> Passwords
* If the interface errors out this patch did not work.