Add functionality to update password with password hash (#15343)

* Add functionality to update password with password hash

This can be used to update a user's password with a direct password
hash instead of providing the password in plaintext.

* Use test helper for local mode

go.tools.mod

@@ -4,7 +4,7 @@ go 1.14
 
 require (
 	github.com/jstemmer/go-junit-report v0.9.1 // indirect
-	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200821090819-f7569391b4d3 // indirect
+	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20200825115902-a83581379d0a // indirect
 	github.com/philhofer/fwd v1.0.0 // indirect
 	github.com/reflog/struct2interface v0.6.1 // indirect
 	github.com/tinylib/msgp v1.1.2 // indirect