Add option to enable HTTP log (and disable by default)

[lmazuel]

@derekbekoe @johanste
This disables HTTP log by default and add two ways to configure it:

• At the operation level: create_or_update(....., enable_http_log=True)
• At the client level: client.config.enable_http_log=True
  Operation level is most important (i.e. if operation says False and client says True, we don't log)

I would recommend the CLI to add client.config.enable_http_log=True in the generic client factory. And to override it by False for known critical operation if necessary (KeyVault? etc.)

I don't merge until you tell me the CLI is ready.

[codecov-io]

Codecov Report

Merging #86 into master will decrease coverage by 0.19%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master      #86     +/-   ##
=========================================
- Coverage   78.05%   77.85%   -0.2%     
=========================================
  Files         273      273             
  Lines       14298    14300      +2     
=========================================
- Hits        11160    11133     -27     
- Misses       3138     3167     +29

Impacted Files	Coverage Δ
msrest/configuration.py	44% <100%> (+0.75%)	⬆️
msrest/service_client.py	82.18% <60%> (-1.06%)	⬇️
msrest/http_logger.py	13.33% <0%> (-60%)	⬇️

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