manager.py updates reset postgres credentials

[Sam Wilson]

After running manager.py to update a stack the lambda functions with database calls throw errors;

{
  "Status": "Failure",
  "Message": "FATAL:  password authentication failed for user \"lambda\"\nFATAL:  password authentication failed for user \"lambda\"\n"
}

[danb]

resolve #1

Why are we doing this? "default" values in template count as 'passing parameters' on stack update operations. That means that your previously perfectuly functional stack will suddenly stop working as your database credentials are zeroed out. Amazing.

Also prevents manager.py from passing 'None' values for some params which will probably also clobber some stuff tbh.

→ <<cset 0309903ccdfb>>