tuf: Add flag to disable encryption
tuf: Add flag to disable encryption

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

theupdateframework/go-tuf,diogomonica/gotuf,endophage/gotuf,runcom/gotuf,cyli/gotuf,dmcgowan/gotuf