[Core] Use dbname from doctrine connection to allow custom values

[vntw]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no

This change makes it possible to use the same database name in every environment. Currently, the InstallDatabaseCommand will always prepend _%env% to the database name, even if I specified sylius (no _%env%) in my dev config, which won't match the expectations made here.

It will return NULL for SQLite and eventually explode since listDatabases is not supported anyway.

[pjedrzejewski]

Thank you @venyii! 👍