Hide key field in edit Flavor Extra Specs dialog

Nova allow user to add key/value to flavor, but its api is only
able to update value according to key.

Hide the key field in edit dialog, in order to avoid confusion.
User can delete and create new one when need to update "key".

Change-Id: Ibd13bfe8a3832d019d16a39b40f643b860e3c612
Closes-Bug: #1257882
Merge "Hide key field in edit Flavor Extra Specs dialog"