add simplified attribute in registry config

[gaoxinge]

What this PR does:

• add simplified attribute in registry config

Special notes for your reviewer:

Adding a simplified attribute in registry config is for:

• Firstly, there is a simplified attribute in java version.

• Secondly, simplified provider urls can release the pressure of the registry center. And we can move other parameters in provider ursl to metedata center and config center.

The code follows the java version, but not strictly.

Does this PR introduce a user-facing change?:

NONE

[zouyx]

Travis still failing

[codecov-io]

Codecov Report

Merging #474 into develop will increase coverage by 0.04%.
The diff coverage is 47.36%.

@@             Coverage Diff             @@
##           develop     #474      +/-   ##
===========================================
+ Coverage    66.45%   66.50%   +0.04%     
===========================================
  Files          184      184              
  Lines         9683     9696      +13     
===========================================
+ Hits          6435     6448      +13     
- Misses        2607     2610       +3     
+ Partials       641      638       -3     

Impacted Files	Coverage Δ
common/url.go	67.27% <42.85%> (-3.89%)	⬇️
config/registry_config.go	78.94% <50.00%> (-1.06%)	⬇️
registry/protocol/protocol.go	80.80% <66.66%> (-0.48%)	⬇️
protocol/dubbo/client.go	69.09% <0.00%> (+1.21%)	⬆️
config_center/nacos/client.go	57.26% <0.00%> (+1.70%)	⬆️
cluster/cluster_impl/failback_cluster_invoker.go	80.64% <0.00%> (+2.15%)	⬆️
filter/filter_impl/hystrix_filter.go	72.03% <0.00%> (+3.38%)	⬆️
config_center/nacos/facade.go	62.50% <0.00%> (+25.00%)	⬆️

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[fangyincheng]

LGTM