Provide a single stunnel connect_host

At present connect_host is specified by each port, individually, as
the same value. Move connect_host to be a direct child of the stunnel
element so it is only specified once.

Although previously we could theoretically specify a different
connect_host for each service, in practice they were the same and
that never would have worked.

This change means Mustache like {{#stunnel.connect_host}} will work.

Change-Id: I25c4bb09cf28a3728e959d4dd583af26a602ad90
Partial-Bug: #1391926