In a migration spec, a default ApplicationSetting object may fail to
create due to an unknown attribute and fall back to
FakeApplicationSetting.  FakeApplicationSetting appears to be missing a
number of methods that are needed
(https://gitlab.com/gitlab-org/gitlab-ce/issues/47491). This commit adds
the ones needed to make an EE spec pass.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7543