Allow presets for message bodies
This lets users of simple_message_to_bus predefine items which should be
present in all message bodies this way:
export MSGBODY_PRESET="key1=value1 key2=value2"
This doesn't work with spaces in either keys or values, any quotation
will be used verbatim.
Signed-off-by: Nils Philippsen <nils@redhat.com>