Modify ftest to support Oracle 12c "pluggable" databases.

In Oracle 12c, when a database is created in a "pluggable" database, new usernames have to start with "c##" for some weird reason.
This restraint can be bypassed with the undocumented "_oracle_script" parameter.

Given how ftest currently works, it's not possible to systematically add "c##" at the beginning of all the oracle usernames (that's not a valid name on "non-pluggable" databases, and ftest has no way to know which kind it's hitting).
-> use "_oracle_script" parameter everywhere instead, and keep the usernames as is.