Engine/GCTScript: Refactor script manager (#580)

* refactor script manager

* remove singleton GCTScriptConfig
* create constant for ".gct" extension
* move GctScriptManager into vm package
* reduce script manager global dependencies
* use manager struct to store runtime override values
* enable/disable scripting subsystem now doesn't store the setting in
config (aligned with other subsystems)
* setting max VMs via start option doesn't change config

* instantiate scriptmanager as part of creating a new Engine

    * script manager config is now set during instantiation
    * run script manager when enabled in conf or explicitly enabled
    * use the Started() method to check if script manager is running

* in tests set script manager as running

* script manager adjustments

* create manager before attempting overrides
* check for nil config when creating script manager

* fix script manager waitgroup counter increased too late

* move autoload() function to autoload.go
* add tests to script manager

engine/subsystem/subsystem.go

@@ -0,0 +1,20 @@
+package subsystem
+
+const (
+	// ErrSubSystemAlreadyStarted message to return when a subsystem is already started
+	ErrSubSystemAlreadyStarted = "manager already started"
+	// ErrSubSystemAlreadyStopped message to return when a subsystem is already stopped
+	ErrSubSystemAlreadyStopped = "already stopped"
+	// ErrSubSystemNotStarted message to return when subsystem not started
+	ErrSubSystemNotStarted = "not started"
+
+	// MsgSubSystemStarting message to return when subsystem is starting up
+	MsgSubSystemStarting = "manager starting..."
+	// MsgSubSystemStarted message to return when subsystem has started
+	MsgSubSystemStarted = "started."
+
+	// MsgSubSystemShuttingDown message to return when a subsystem is shutting down
+	MsgSubSystemShuttingDown = "shutting down..."
+	// MsgSubSystemShutdown message to return when a subsystem has shutdown
+	MsgSubSystemShutdown = "manager shutdown."
+)