allow configurable Postgres authentication

[donsizemore]

to allow the Python installer to behave more closely with the guides' manual specification.

when db.postgres.auth is set to trust don't create the database, the user or set permissions on the public schema.

will also allow for configurable authentication, which will be nice. default to scram-sha-256

[donsizemore]

setting db.postgres.auth to trust results in

postgres=# \l
                                   List of databases
   Name    |  Owner   | Encoding |   Collate   |    Ctype    |    Access privileges     
-----------+----------+----------+-------------+-------------+--------------------------
 postgres  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | 
 <snip>
 vagrantdb | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =Tc/postgres            +
           |          |          |             |             | postgres=CTc/postgres   +
           |          |          |             |             | vagrantuser=CTc/postgres
(4 rows)