Check whether sudo is enabled in BaseSudoTestCase.setUpClass

Check whether sudo is enabled in BaseSudoTestCase.setUpClass

Currently functional test classes requiring invocation of commands via a
root helper inherit from BaseSudoTestCase[1] and should call explicitly
check_sudo_enabled in order to skip the test if sudo is disabled[2].

This change performs such check during BaseSudoTestCase.setUp in order
to ensure the check is done and replaces parent class for functional
test cases not requiring invocation of commands via a root helper.

This change prepares to transform BaseOVSLinuxTestCase[3] methods into
helper methods.

[1] in neutron.tests.functional.base
[2] sudo is enabled if OS_SUDO_TESTING is set to '1' or 'True'
[3] in neutron.tests.functional.agent.linux.base

Change-Id: I91a3506524352211729ac8b3b07cf3082fd8f13a