secret: Use consistent naming for variables
    
    When processing a virSecretPtr use 'secret' as a variable name.
    
    When processing a virSecretObjPtr use 'obj' as a variable name.
    
    When processing a virSecretDefPtr use 'def' as a variable name,
    unless a distinction needs to be made with a 'newdef' such as
    virSecretObjListAddLocked (which also used the VIR_STEAL_PTR macro
    for the configFile and base64File).
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>