Support password generation

[huyhg]

• We will keep template expansion limited to environment variable substitution, essentially what envsubst is intended to do.
• In order to generate passwords, we will do a 2-pass approach. The first pass is to generate passwords and set them onto the password env variables. The second pass is envsubst-ing.
• Find a way to use pwgen.

[huyhg]

The password env variables should be defined and used in the same manner as other parameters. Specifically, they would be defined in the schema file and have special annotation:

properties:
  myPassword:
    type: string
    x-googleProperty:
      type: GENERATED_PASSWORD
      generatedPassword:
        length: 10

Because parameters are being passed to the deployer container as mounted volume, the UI can create a Secret (instead of a ConfigMap) containing the generated password values and mount it onto the deployer container. The UI, however, does not need to generate these properties as they are handled by the deployer if not explicitly specified.

On a related note, if the deployer is handling generation of passwords, it makes sense for it to be capable of handling default values.

[huyhg]

@deustis @trironkk @vcanaa @khajduczenia