Only set the ipv6only listen option on the default server

Apparently some nginx listen socket options can only be defined
in a single server server. Let's (ab)use the default_server
parameter to establish that.

This implies that on all localssl terminators, at least one
server needs to be have default_server => true. The notify
resource and the corresponding dependency attempt to ensure
this.

Change-Id: I2f0c61ac28497d4e52cc7da7a1c33fc00c04a67a
Merge "Only set the ipv6only listen option on the default server" into production