virttest.data_dir: Add test provider dir APIs

We'll frequently need to locate providers, so this
provides an appropriate API to do that.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.asset: Add test provider related APIs

Create an API to read from all test provider resources,
and return a list of dicts with information, that will
be used to download them. Also, add APIs to download
said test providers.

Use the autotest API autotest.client.test_config, so we
can make the functions more concise. Take the time to
make get_asset_info() shorter using this API.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
run: Add --update providers option

Instead of having test providers to be updated every
time the suite executes, update them only if asked to.

Introduce the needed hooks in the bootstrap function to
have the test providers installed on the 1st virt-test
execution.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>