Create database named with the variable EXPOSURE_DB instead of POSTGRES_DB

We need an exposure database that is not the same database as the postgres database. This first we want to have our exposure schemas in and the second we want to use for 'administrative' things (creating new databases, creating dumps, etc.). For example, in exposurelib_tests we could copy the exposure database template to a testing exposure database, to ensure we start with a clean database each time.

\fyi @all