Add per-service env vars for *_SERVICE_HOST

As a replacement of a single SERVICE_HOST variable, offer a FOO_SERVICE_HOST
variable.  This will help ease the transition to ip-per-service, where there
is no longer a single service host.

# *** ERROR: *** Some files are missing the required boilerplate
# header from hooks/boilerplate.txt:
#   examples/guestbook/redis-slave/run.sh
#
# Your commit will be aborted unless you fix these.
#   COMMIT_BLOCKED_ON_BOILERPLATE