Fix Postgres name + optionally grant postgres_exporter role to deployer role

[endofcake]

• PostgreSQL name was misspelled in 53b9d9c
• If the deployer role is not superuser (which is the case in AWS RDS,
  for example), we need to grant it access to postgres_exporter role
  before we can create a schema with AUTHORIZATION postgres_exporter.

[coveralls]

Coverage remained the same at 50.801% when pulling 92a4dfc on endofcake:master into 3b9a688 on wrouesnel:master.