Rename global_config to settings

For a long time, we have been using a quite cumbersome
way of accessing settings in the autotest global config
file. In nearly all source code files that do this
access, people created shortcuts to avoid accessing the
full API namespace, such as:

from autotest.client.shared import global_config

c = global_config.global_config.get_config_value
c.get_value("mysection", "mykey")

This is an eyesore. Let's change this to something
more sane:

from autotest.client.shared.settings import settings

settings.get_value("mysection", "mykey")

CC: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>