backend: Move postgresql backend initialization in a separate class

Since commit swh-core@89d48572, the "swh db init" command attempts
to instantiate the postgresql backend of a swh module using the get_datastore
function of the module with a "cls" and "db" parameters.

While it works fine for most of swh modules, it does not for the vault as more
parameters are expected by the "get_vault" function and thus the database init
is failing. The issue was spotted in the docker environment after updating the
swh/stack image.

So extract postgresql backend initialization for the vault in a new class named
VaultBackendDataStore and set it as swh.vault.get_datastore attribute value.