Puppet 7 is reaching its EOL very soon. Enable testing with Puppet 8
to prepare complete migration to Puppet 8. Note that litmus jobs are
kept non-voting in this step but will later be made voting.
Change-Id: I4578a2265f962ec38b7bddb19e0dfbb624d117de
(cherry picked from commit ed57a60dcc)
(cherry picked from commit 65af6d5d14)
Takashi Kajinami
dc5ae3a723