Add init parameter to support unifying web context name in Sentinel Web CommonFilter

[zhaoyuguang]

resolve #1024

CommonFilter.java 增加可以控制context是统一名称或是为url的自定义配置参数

[sczyh30]

The CI indicates test failure: https://travis-ci.org/alibaba/Sentinel/builds/601795740?utm_source=github_status&utm_medium=notification

Could you please resolve it?

[codecov-io]

Codecov Report

Merging #1111 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1111      +/-   ##
============================================
+ Coverage     42.81%   42.88%   +0.07%     
- Complexity     1495     1499       +4     
============================================
  Files           320      320              
  Lines          9384     9387       +3     
  Branches       1278     1280       +2     
============================================
+ Hits           4018     4026       +8     
+ Misses         4871     4868       -3     
+ Partials        495      493       -2

Impacted Files	Coverage Δ	Complexity Δ
...aba/csp/sentinel/adapter/servlet/CommonFilter.java	85.71% <100%> (+1.09%)	12 <0> (+2)	⬆️
...tinel/slots/block/flow/param/ParamFlowChecker.java	56.08% <0%> (+3.37%)	30% <0%> (+2%)	⬆️

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[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!