oeqa/selftest/imagefeatures: fix SSH without password tests
* We need to set EXTRA_IMAGE_FEATURES outright or existing values will
affect the test
* For test case 1107 we need "empty-root-password" to match the behaviour
described in the test case
* For test case 1115 we shouldn't be able to connect as root with the
features we are setting
Test cases 1107 and 1115 have been updated in Testopia to match.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
