fleet: refactor config

- Explicitly specify all of the valid options for fleet
- Seperate the config from Units()
- Add YAML tags for the fields

config/fleet.go

@@ -0,0 +1,14 @@
+package config
+
+type Fleet struct {
+	AgentTTL                string `yaml:"agent-ttl"                 env:"FLEET_AGENT_TTL"`
+	EngineReconcileInterval string `yaml:"engine-reconcile-interval" env:"FLEET_ENGINE_RECONCILE_INTERVAL"`
+	EtcdCAFile              string `yaml:"etcd-cafile"               env:"FLEET_ETCD_CAFILE"`
+	EtcdCertFile            string `yaml:"etcd-certfile"             env:"FLEET_ETCD_CERTFILE"`
+	EtcdKeyFile             string `yaml:"etcd-keyfile"              env:"FLEET_ETCD_KEYFILE"`
+	EtcdRequestTimeout      string `yaml:"etcd-request-timeout"      env:"FLEET_ETCD_REQUEST_TIMEOUT"`
+	EtcdServers             string `yaml:"etcd-servers"              env:"FLEET_ETCD_SERVERS"`
+	Metadata                string `yaml:"metadata"                  env:"FLEET_METADATA"`
+	PublicIP                string `yaml:"public-ip"                 env:"FLEET_PUBLIC_IP"`
+	Verbosity               string `yaml:"verbosity"                 env:"FLEET_VERBOSITY"`
+}