This commit adds new settings for the ssl keystore (not the ES keystore)
passphrase settings. New setting names are used, instead of trying to
support the existing names in both yml and the ES keystore, so that
there does not need to be complicated logic between the two. Note that
the old settings remain the only way to set the ssl passphrases for the
transport client, but the Settings object for transport clients are
created in memory by users, so they are already as "secure" as having a
loaded ES keystore. Also note that in the long term future (6.x
timeframe?) these settings should be deprecated and the keys/certs
themselves should be moved into the ES keystore, so there will be no
need for separate keystores/passphrases.
relates elastic/elasticsearch#22475
Original commit: elastic/x-pack-elasticsearch@be5275fa3d
9b3fb66394