[TEST] wait for yellow to avoid searching while relocating

After upgrading shard might start relocating again. If there are no
replicas the cluster state of a node might not be up to data for
a few miliseconds and direct a search request to a node that does not
have the shard anymore. This result in the following test failures:

1> java.lang.AssertionError: Count is 99 but 101 was expected.  Total shards: 13 Successful shards: 12 & 0 shard failures:
1>     __randomizedtesting.SeedInfo.seed([1932F73B458703CA:6F4FAD3DAC55591C]:0)
1>     [...org.junit.*]
1>     org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount(ElasticsearchAssertions.java:184)
1>     org.elasticsearch.bwcompat.BasicBackwardsCompatibilityTest.testIndexRollingUpgrade(BasicBackwardsCompatibilityTest.java:358)

Waiting for relocation finished should fix this.
Docs: use https to download the gpg public key

Closes #8818
[TEST] Reset test logger to default level
[ENGINE] Fix updates dynamic settings in InternalEngineHolder

After the refactoring in #8784 some settings didn't get passed to the
actual engine and there exists a race if the settings are updated while
the engine is started such that the actual starting engine doesn't see
the latest settings. This commit fixes the concurrency issue as well as
adds tests to ensure the settings are reflected.