okta-kubectl-auth crashes without any parameters

[simonswine]

We should verify that base-domain, client-id and client-secret are supplied and valid

./okta-kubectl-auth 
panic: Failed to query provider "": Get /.well-known/openid-configuration: unsupported protocol scheme ""

goroutine 1 [running]:
github.com/jetstack/okta-kubectl-auth/pkg/okta.(*Okta).provider(0xc420154000, 0xc420132340)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/pkg/okta/okta.go:124 +0x20f
github.com/jetstack/okta-kubectl-auth/pkg/okta.(*Okta).offlineAsScope(0xc420154000, 0xc4200ca240)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/pkg/okta/okta.go:139 +0x49
github.com/jetstack/okta-kubectl-auth/pkg/okta.(*Okta).Authorize(0xc420154000, 0x0, 0x834c50, 0x4)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/pkg/okta/okta.go:176 +0xcc
github.com/jetstack/okta-kubectl-auth/cmd.glob..func1(0xa785e0, 0xa9b1a8, 0x0, 0x0, 0x0, 0x0)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/cmd/root.go:29 +0x48
github.com/jetstack/okta-kubectl-auth/vendor/github.com/spf13/cobra.(*Command).execute(0xa785e0, 0xc4200a6190, 0x0, 0x0, 0xa785e0, 0xc4200a6190)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/vendor/github.com/spf13/cobra/command.go:756 +0x468
github.com/jetstack/okta-kubectl-auth/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xa785e0, 0x19, 0xc42006ff58, 0x75be71)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/vendor/github.com/spf13/cobra/command.go:846 +0x30a
github.com/jetstack/okta-kubectl-auth/vendor/github.com/spf13/cobra.(*Command).Execute(0xa785e0, 0xc420096058, 0x0)
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/vendor/github.com/spf13/cobra/command.go:794 +0x2b
github.com/jetstack/okta-kubectl-auth/cmd.Execute()
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/cmd/root.go:39 +0x2d
main.main()
	/home/christian/.golang/packages/src/github.com/jetstack/okta-kubectl-auth/main.go:8 +0x20