dovecot: optionally use clucene.

References #26100.
Closes #26102.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
dovecot: cleanup caveats.
percona-server 5.6.15-rel63.0

Closes #26131.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
node: stable version is allowed to be built with python 2.6