Double-quote value of synchronous_standby_names in rep_mode.conf

PostgreSQL 9.6 introduced a new syntax for specifying
synchronous_standby_names. The old syntax, used by the pgsql RA, is
still valid but PostgreSQL now treats the standby-names in
synchronous_standby_names as SQL identifiers. This means such values
need to be double-quoted since they can easily contain dashes or other
characters that are not valid in a bare SQL identifier.

See the docs for synchronous_standby_names in
https://www.postgresql.org/docs/9.6/static/runtime-config-replication.html
for confirmation and
https://www.postgresql.org/message-id/21183.1481253534%40sss.pgh.pa.us
for a short discussion.