Add a sysview call to determine if a mount is local. (#3899)

This is useful for deciding when to run upgrade logic, e.g. if on a
performance secondary but local it's fine to run.

logical/system_view.go

@@ -39,6 +39,10 @@ type SystemView interface {
 	// despite known slowdowns.
 	CachingDisabled() bool
 
+	// When run from a system view attached to a request, indicates whether the
+	// request is affecting a local mount or not
+	LocalMount() bool
+
 	// ReplicationState indicates the state of cluster replication
 	ReplicationState() consts.ReplicationState
 
@@ -63,6 +67,7 @@ type StaticSystemView struct {
 	CachingDisabledVal  bool
 	Primary             bool
 	EnableMlock         bool
+	LocalMountVal       bool
 	ReplicationStateVal consts.ReplicationState
 }
 
@@ -86,6 +91,10 @@ func (d StaticSystemView) CachingDisabled() bool {
 	return d.CachingDisabledVal
 }
 
+func (d StaticSystemView) LocalMount() bool {
+	return d.LocalMountVal
+}
+
 func (d StaticSystemView) ReplicationState() consts.ReplicationState {
 	return d.ReplicationStateVal
 }