Add option do disable log payloads indexation

[brasseld]

When the logging condition is true, the gateway is storing:

• Request headers + payload
• Response headers + payload

Since #2053 the payload is defined as a keyword for being able to search logs base on payloads.

This results with issues for large payloads:

09:19:30.692 [vert.x-eventloop-thread-0] [] ERROR i.g.e.client.http.HttpClient - An error occurs while indexing data into ES: indice[gravitee-log-2019.11.13] error[Document contains at least one immense term in field="client-response.body" (whose UTF8 encoding is longer than the max length 32766), all of which were skipped.  Please correct the analyzer to not produce such terms.  The prefix of the first immense term is: '[123, 10, 32, 34, 101, 120, 112, 101, 114, 116, 79, 112, 105, 110, 105, 111, 110, 115, 34, 58, 32, 91, 10, 32, 123, 10, 32, 34, 101, 120]...', original message: bytes can be at most 32766 in length; got 49023]

We should provide a way to enable / disable payload indexation