[RFC] 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")

In order to keep the config file name in sync with
the library name, and also, have a nicer name, we
renamed:

global_config.ini -> settings.ini
shadow_config.ini -> shadow.ini

This last change might bring some headache for
current users that are using older versions of
autotest and want to upgrade, but this file
should be easily rebaseable.

CC: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>