ENYO-2591: Delete the "None" element for the signing keys picker.

ENYO-2591 : 
Since it's not possible to disjoint a signing key associated to a platform from a Phonegap user, when a signing key is created it's no longer possible to build an application without selecting a signing key. To make the Phonegap build UI coherent with this constraint the "None" element is discarded from the Signing keys picker.
Also, the signing keys row is arranged vertically to be properly displayed.
In the case where there is no signing key associated with a platform in the user account, the message "Ne signing keys for this platform" is displayed instead of having an empty signing keys picker.

Ready for review.
Tested on Chrome (Windows 7)
Enyo-DCO-1.1-Signed-off-by: Rafik Adiche mahmoud-rafik.adiche@hp.com.