Fix backend lint issues

[mikhail-vl]

exec: golangci-lint run ./...
pkg/grafana-redis-datasource.go:69:6: ineffectual assignment to `status` (ineffassign)
	var status = backend.HealthStatusUnknown
	    ^
pkg/commands.go:137:9: S1034: assigning the result of this type assertion to a variable (switch result := result.(type)) could eliminate the following type assertions:
	pkg/commands.go:140:12
	pkg/commands.go:147:19
	pkg/commands.go:163:12 (gosimple)
	switch result.(type) {
	       ^
pkg/commands.go:172:11: S1034: assigning the result of this type assertion to a variable (switch value := value.(type)) could eliminate the following type assertions:
	pkg/commands.go:174:36 (gosimple)
			switch value.(type) {
			       ^
pkg/commands.go:180:13: S1034: assigning the result of this type assertion to a variable (switch element := element.(type)) could eliminate the following type assertions:
	pkg/commands.go:182:38
	pkg/commands.go:184:31 (gosimple)
					switch element.(type) {
					       ^
pkg/commands.go:311:9: S1034: assigning the result of this type assertion to a variable (switch result := result.(type)) could eliminate the following type assertions:
	pkg/commands.go:313:31 (gosimple)
	switch result.(type) {
	       ^
pkg/commands.go:777:11: S1034: assigning the result of this type assertion to a variable (switch arg := arg.(type)) could eliminate the following type assertions:
	pkg/commands.go:779:34
	pkg/commands.go:781:23
	pkg/commands.go:783:16 (gosimple)
			switch arg.(type) {
			       ^
Error: running "golangci-lint run ./..." failed with exit code 1

Exited with code exit status 1
CircleCI received exit code 1

[mikhail-vl]

Looks good!