Add maven-clean-plugin for hubble

[simon824]

Avoid creating too many temporary files by cp and add maven-clean-plugin for hubble to clean package files.

[codecov]

Codecov Report

Merging #335 (84d8f86) into master (6d82b15) will increase coverage by 5.26%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #335      +/-   ##
============================================
+ Coverage     67.43%   72.69%   +5.26%     
- Complexity      878     1850     +972     
============================================
  Files            86      242     +156     
  Lines          4029     8109    +4080     
  Branches        477      719     +242     
============================================
+ Hits           2717     5895    +3178     
- Misses         1107     1830     +723     
- Partials        205      384     +179     

Impacted Files	Coverage Δ
.../main/java/com/baidu/hugegraph/api/job/JobAPI.java	100.00% <0.00%> (ø)
...in/java/com/baidu/hugegraph/structure/Element.java	37.50% <0.00%> (ø)
.../baidu/hugegraph/structure/constant/IndexType.java	73.33% <0.00%> (ø)
...n/java/com/baidu/hugegraph/api/auth/AccessAPI.java	100.00% <0.00%> (ø)
.../com/baidu/hugegraph/structure/traverser/Kout.java	100.00% <0.00%> (ø)
...a/com/baidu/hugegraph/structure/SchemaElement.java	94.11% <0.00%> (ø)
...n/java/com/baidu/hugegraph/driver/TaskManager.java	28.57% <0.00%> (ø)
.../hugegraph/exception/InvalidResponseException.java	0.00% <0.00%> (ø)
...aidu/hugegraph/structure/constant/Cardinality.java	69.23% <0.00%> (ø)
...du/hugegraph/structure/traverser/CountRequest.java	75.00% <0.00%> (ø)
... and 146 more

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[imbajin]

thanks, shall we put the plugin in root's pom?

[simon824]

thanks, shall we put the plugin in root's pom?

but seems different subprojects have different files to clean, maybe it's better to separate?