This patch modifies the multiplexer logic to support full recursive
multiplexation, unlike the old version which only flattened the
multiplexed nodes. This allows greater flexibility.

Additionally it removes the !join flag and inverts the logic to use !mux
instead. By default leaves are not multiplexed and all are part of the
current params. Users can specify exact nodes which get multiplexed by
using !mux keyword.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>