Add in autogenerated passwords

Any string NEPHO_CHANGEME_PASSWORD in the template will be replaced with an autogenerated password. Currently there's one per entire template, but since we only use this right now for RDS, not an issue.

In the future we'll want to replace each instance with a different password, and use the CF templating to handle refs within the template.

Passwords currently are 32 chars, start with a-zA-Z and the rest is a-zA-z0-9 (i.e. no digits in the first char.)