Add a content parameter to ferm::conf

It might well be possible we want to pass template to ferm::conf. This
commit add the content parameter which works like in the file resource
(in fact it is passed directly to the corresponding file resource). Some
checks are also added to avoid passing both content and source as a
parameter

Change-Id: I7b61391c34e25dce2089d063e019eac03adf4f46
Merge "Add a content parameter to ferm::conf" into production