Fix VM provider get_available_providers() test

Fix VM provider get_available_providers() to be able to add new
providers without changing it.

Change-Id: I01167cfe21a2ff5e74bf875649273fef4aa1df03
db.db_create() shouldn't call hiddenly db.db_drop()

It is nasty things that could produce a lot of problems.

Change-Id: I8a04c70817a9b975d6064693a2d27597c4b54d3e
Add CLI utils for DB recreate

We should be able to init DB for rally from CLI

blueprint db-task-track

Change-Id: I52804e86b24d0e9334687d2545ada9a6a00595de