[TEST] randomly introduced a client node within test cluster

The default number of clients nodes is randomized between 0 and 1, applied to all cluster scopes (global, suite and test). Can be changed through the newly added `@ClusterScope#numClientNodes`.

In our tests we currently refer to nodes in a generic way. All the tests that either stop or start nodes rely on the fact that those nodes hold data though. Made that clearer as that becomes more important when introducing other types of nodes within the test cluster. Reflected this by adapting and renaming the following methods in `TestCluster`:

- ensureAtLeastNumNodes to ensureAtLeastNumDataNodes
- ensureAtMostNumNodes to ensureAtMostNumDataNodes
- stopRandomNode to stopRandomDataNode

and the following ones in `ElasticsearchIntegrationTest`:

- allowNodes to allowDataNodes
- dataNodes to numDataNodes.
- @ClusterScope#numNodes to numDataNodes
- @ClusterScope#minNumNodes to minNumDataNodes
- @ClusterScope#maxNumNodes to maxNumDataNodes

Added facilities to be able to deal with data nodes specifically, like for instance retrieve a client to a data node, or retrieve an instance of a class through guice only from data nodes.

Adapted existing tests to successfully run although there's a node client around.

Fixed _cat/allocation REST tests to make disk.total, disk.avail and disk.percent optional as client nodes won't return that info.

Closes #5949
[TEST] made sure nodeSettings method gets called for every node type, not only data nodes in case numDataNodes is specified.

This fixes a test ZenUnicastDiscoveryTests when running in network mode
Use URI vs URL accessing File from classpath.

URL escapes special characters such as spaces which
causes the resource to not be found when used to create
a File object.  Use URI.

Closes #5915
Added global ordinals terms aggregator impl that is optimized low cardinality fields.
Instead of resolving the global ordinal for each hit on the fly, resolve the global ordinals during post collect.
On fields with not so many unique values, that can reduce the number of global ordinals significantly.

Closes #5895
Closes #5854
Cut other aggregations over to use collectExistingBucket() if a bucket ord has been hit, that already exists.

Closes #5955
Cut p/c queries (has_child and has_parent queries) over to use global ordinals instead of being bytes values based.

Closes #5846
Added AwaitsFix
Aggregations parsing is too lenient.

Close #5827