(#2112) Ensure CHORIA_SRV_DOMAIN is actually getting used

The logic to pull config defaults from an environment variable is only used when a config has a default.
This adds empty string default for SRV domain which will trigger the logic to pull a default value also from the CHORIA_SRV_DOMAIN environment variable.

config/choria.go

@@ -22,7 +22,7 @@ type ChoriaPluginConfig struct {
 	PuppetDBHost     string `confkey:"plugin.choria.puppetdb_host" default:""`                                                                // The host hosting your PuppetDB, used by the "choria" discovery plugin
 	PuppetDBPort     int    `confkey:"plugin.choria.puppetdb_port" default:"8081"`                                                            // The port your PuppetDB listens on
 	UseSRVRecords    bool   `confkey:"plugin.choria.use_srv" default:"true" url:"https://choria.io/docs/deployment/dns/"`                     // If SRV record lookups should be attempted to find Puppet, PuppetDB, Brokers etc
-	SRVDomain        string `confkey:"plugin.choria.srv_domain" environment:"CHORIA_SRV_DOMAIN" url:"https://choria.io/docs/deployment/dns/"` // The domain to use for SRV records, defaults to the domain the server FQDN is in
+	SRVDomain        string `confkey:"plugin.choria.srv_domain" environment:"CHORIA_SRV_DOMAIN" default:"" url:"https://choria.io/docs/deployment/dns/"` // The domain to use for SRV records, defaults to the domain the server FQDN is in
 
 	Provision            bool `confkey:"plugin.choria.server.provision" default:"false" url:"https://choria-io.github.io/provisioner/"`              // Specifically enable or disable provisioning
 	ProvisionAllowUpdate bool `confkey:"plugin.choria.server.provision.allow_update" default:"false" url:"https://choria-io.github.io/provisioner/"` // Allows the provisioner to perform in-place version updates