Make wikimetrics' database setup depend on alembic

The setup procedure documented for wikimetrics fails with:

  err: /Stage[main]/Wikimetrics::Database/Exec[alembic_upgrade_head]/returns: change from notrun to 0 failed: Could not find command '/usr/local/bin/alembic'

. Hence we now make that step depend on
install_wikimetrics_dependencies, so puppet makes sure alembic is
available to setup the database. Thereby, the above error no longer
occurs.

To keep the order of operations, we move the ::wikimetrics::database
after its requirements.

Change-Id: I23c357c6864738fd676680e2b85bd5d1f2a927de