command/agent: added missing syslog wrapper

Previously, we would write a service-defaults config regardless of the
value of the -default-protocol flag or the Pod's protocol annotation. If
neither of these were set, we would write a config with protocol set to
the empty string. This is the same as setting it to tcp. So for every
service, we were by default setting its protocol to tcp.

If a user explicitly created a global proxy-defaults config and set the
protocol to, say, http. Then our service-defaults config would override
that protocol. This behaviour was unexpected because users had never
explicitly told us to write a service-defaults config and it wasn't
helping them.

This change will cause us to only write a service-defaults config if the
protocol is explicitly set–either via the -default-protocol flag or via
the Pod annotation.

I've also renamed the CentralConfig field to WriteServiceDefaults
because there are many types of central config now so it's best to be
explicit.

Fixes hashicorp#168