Docs: fix formula typo in documentation of the gauss decay in function_score

closes #7506
[Rest] Add the cluster name to the "/" endpoint

The root endpoint returns basic information about this node, like it's name and ES version etc. The cluster name is an important information that belongs in that list.

Closes #7524
mappings: keep parameters in mapping for _timestamp, _index and _size even if disabled

Settings that are not default for _size, _index and _timestamp were only build in
toXContent if these fields were actually enabled.
_timestamp, _index and _size can be dynamically enabled or disabled.
Therfore the settings must be kept, even if the field is disabled.
(Dynamic enabling/disabling was intended, see TimestampFieldMapper.merge(..)
and SizeMappingTests#testThatDisablingWorksWhenMerging
but actually never worked, see below).

To avoid that _timestamp is overwritten by a default mapping
this commit also adds a check to mapping merging if the type is already
in the mapping. In this case the default is not applied anymore.
(see
SimpleTimestampTests#testThatUpdatingMappingShouldNotRemoveTimestampConfiguration)

As a side effect, this fixes
- overwriting of paramters from the _source field by default mappings
  (see DefaultSourceMappingTests).
- dynamic enabling and disabling of _timestamp and _size ()
  (see SimpleTimestampTests#testThatTimestampCanBeSwitchedOnAndOff and
  SizeMappingIntegrationTests#testThatTimestampCanBeSwitchedOnAndOff )

Tests:

Enable UpdateMappingOnClusterTests#test_doc_valuesInvalidMappingOnUpdate again
The missing settings in the mapping for _timestamp, _index and _size caused a the
failure: When creating a mapping which has settings other than default and the
field disabled, still empty field mappings were built from the type mappers.
When creating such a mapping, the mapping source on master and the rest of the cluster
can be out of sync for some time:

1. Master creates the index with source _timestamp:{_store:true}
   mapper classes are in a correct state but source is _timestamp:{}
2. Nodes update mapping and refresh source which then completely misses _timestamp
3. After a while source is refreshed again also on master and the _timestamp:{}
   vanishes there also.

The test UpdateMappingOnCusterTests#test_doc_valuesInvalidMappingOnUpdate failed
because the cluster state was sampled from master between 1. and 3. because the
randomized testing injected a default mapping with disabled _size and _timestamp
fields that have settings which are not default.

The test
TimestampMappingTests#testThatDisablingFieldMapperDoesNotReturnAnyUselessInfo
must be removed because it actualy expected the timestamp to remove
parameters when it was disabled.

closes #7137
function_score: add optional weight parameter per function

Weights can be defined per function like this:

```
"function_score": {
    "functions": [
        {
            "filter": {},
            "FUNCTION": {},
            "weight": number
        }
        ...
```
If `weight` is given without `FUNCTION` then `weight` behaves like `boost_factor`.
This commit deprecates `boost_factor`.

The following is valid:

```
POST testidx/_search
{
  "query": {
    "function_score": {
      "weight": 2
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "weight": 2
        },
        ...
      ]
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "FUNCTION": {},
          "weight": 2
        },
        ...
      ]
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "filter": {},
          "weight": 2
        },
        ...
      ]
    }
  }
}
POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "filter": {},
          "FUNCTION": {},
          "weight": 2
        },
        ...
      ]
    }
  }
}
```

The following is not valid:

```
POST testidx/_search
{
  "query": {
    "function_score": {
      "weight": 2,
      "FUNCTION(including boost_factor)": 2
    }
  }
}

POST testidx/_search
{
  "query": {
    "function_score": {
      "functions": [
        {
          "weight": 2,
          "boost_factor": 2
        }
      ]
    }
  }
}
````

closes #6955
closes #7137
fix typo in class name
[TEST] Unify the randomization logic for number of shards and replicas

We currently have two ways to randomize the number of shards and replicas: random index template, that stays the same for all indices created under the same scope, and the overridable `indexSettings` method, called by `createIndex` and `prepareCreate` which returns different values each time.

Now that the `randomIndexTemplate` method is not static anymore, we can easily apply the same logic to both. Especially for number of replicas, we used to have slightly different behaviours, where more than one replicas were only rarely used through random index template, which gets now applied to the `indexSettings` method too (might speed up the tests a bit)

Side note: `randomIndexTemplate` had its own logic which didn't depend on `numberOfReplicas` or `maximumNumberOfReplicas`, which was causing bw comp tests failures since in some cases too many copies of the data are requested, which cannot be allocated to older nodes, and the write consistency quorum cannot be met, thus indexing times out.

Closes #7522
Tests: wait for yellow instead of green