Modifying the bosh Director's manifest is not supported. It will be regenerated quite frequently, so any changes you make will be overwritten. However, you can tell Bosh to not generate random passwords here -> https://docs.pivotal.io/pivotalcf/2-2/customizing/vsphere-config.html#security-config
Pivotal *strongly* recommends that you use randomly generated passwords (i.e. the default behavior).