states/statemgr: Print backend name in lock Info

[radeksimko]

Current Terraform Version

v0.12.0-dev

Use-cases

Debugging, mainly.

Proposal

Terraform currently exposes the following info about a lock:

terraform/states/statemgr/locker.go

Lines 185 to 196 in 0995559

	// String return a multi-line string representation of LockInfo
	func (l *LockInfo) String() string {
	tmpl := `Lock Info:
	ID: {{.ID}}
	Path: {{.Path}}
	Operation: {{.Operation}}
	Who: {{.Who}}
	Version: {{.Version}}
	Created: {{.Created}}
	Info: {{.Info}}
	`

which can make it a bit difficult to even embark on any debugging route as one first has to understand where to look (filesystem or any particular remote storage).

I think it would be helpful to expose the name of the backend which holds the lock, e.g.

Lock Info:
  ID:        a7870da6-a5a2-3e83-54f5-28ace4d5bb7c
  Backend:   s3
  Path:      terraform.tfstate
  Operation: OperationTypeApply
  Who:       radeksimko@local
  Version:   0.12.0
  Created:   2019-01-13 23:02:37.282887 +0000 UTC
  Info: