Update sample properties file

Update the default locale config to also specify the encoding.
Merge pull request #1291 from elyezer/fix-sample-properties

Update sample properties file
Make ActivationKey readable, trim down tests

Make `ActivationKey` inherit from `EntityReadMixin`. Use this mixin to trim down
module `tests.foreman.api.test_activationkey_v2`: replace many usages of
`client.get` with `ActivationKey.read_json`.

All tests pass or are skipped.