[RFR] Fix deprecation warnings for switch_to_alert() and log_message()

[tpapaioa]

This PR resolves two deprecation warnings:

1.) Replace deprecated use of level="warn" with level="warning" in CFMENavigateStep's log_message() method:

cfme/tests/cloud_infra_common/test_retirement.py::test_set_retirement_date[ec2-no_warning]
  /XXX/cfme/utils/appliance/implementations/ui.py:527: DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
    getattr(logger, level)(str_msg)

2.) Replace deprecated use of selenium.switch_to_alert() with selenium.switch_to.alert() in HandleModalsMixin's get_alert() method:

cfme/tests/cloud_infra_common/test_retirement.py::test_set_retirement_date[ec2-no_warning]
  /XXX/cfme/utils/appliance/implementations/common.py:38: DeprecationWarning: use driver.switch_to.alert instead
    return self.selenium.switch_to_alert()

{{ pytest: -v --long-running --use-provider ec2west -k 'test_set_retirement_date[ec2-no_warning]' cfme/tests/cloud_infra_common/test_retirement.py }}