Add random password feature

[mooncube]

We need to manage user's channels and other resources using
Terraform without knowledge of user's actual password (due to
privacy issues).

So the following feature is implemented.

If password field in user resource definition is set to "-", the
user is created with a random password and "reset password" email
is sent to the user.

[ndrpnt]

Hi @mooncube, thanks a lot for your PR. Sorry, for the slow reply, as you can see this repo doesn't see much activity.

I guess the reason for this change is to avoid using a random_password resource that would imply storing the generated password in Terraform state file. I'm not sure that it does much in terms of privacy (we don't guarantee anything to the user and still have ways to know the password), but since you seem to have a use for it I'm willing to merge your PR. Would you mind taking a look at the review? Thanks.

[ndrpnt]

Suggested change

	password.WriteString(
	string(passwordChars.Specials[random.Intn(len(passwordChars.Specials))]),
	)
	// Using crypto/rand
	i, err := rand.Int(rand.Reader, big.NewInt(len(passwordChars.Specials)))
	if err != nil {
	return "", err
	}
	password.WriteString(string(passwordChars.Specials[i.Uint64()]))

[ndrpnt]

I think we should use crypto/rand for password generation. See my suggestion below for an example