virttest: Move utils_koji to staging

This is yet another library that we want to move to
autotest proper, since it is in fact already there.
So it needs to be in virttest.staging rather than virttest.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.staging.utils_koji: Retry file downloads

In order to make the code a little more robust, so
we can achieve better success rates in flaky networking
infrastructures, if we had a failure downloading a file
from Brew or Koji, retry during a timeout (30 seconds),
in fixed time intervals (3 seconds). If at the end of
this timeout we still didn't get it, give up and throw
an error.

As the code uses frequently the terminology 'get' for
this rather than 'download', I opted for an internal
method _get() that encapsulates all the process nicely.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
general: More docstring fixes

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