rabbitmq: quote passwd when adding user/changing passwd

This fixes #6338, enabling the setting of passwords with characters that
would otherwise be interpolated by the shell by single-quoting the
password.
Remove unused arg 'name'

This was likely the result of the function definition being
copied-and-pasted when this function was added, and the unneeded
argument not being removed.
Merge pull request #6421 from terminalmage/issue6338

rabbitmq: quote passwd when adding user/changing passwd