Fixed an issue with openapi item comment being too long。#2823

[Accelerator96]

通过在openapi的接口中限制item comment的长度。

看了下数据库item表下comment字段能放很长。。
https://github.com/ctripcorp/apollo/blob/ec8ed36479202f35edc1ba7cb6d3ac199040f5d6/scripts/db/migration/configdb/V1.0.0__initialization.sql#L215
但是参考前端的长度限制这里就限制64了。

#2823

[codecov-io]

Codecov Report

Merging #2849 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master   #2849      +/-   ##
===========================================
- Coverage     50.41%   50.4%   -0.02%     
  Complexity     2083    2083              
===========================================
  Files           414     414              
  Lines         12668   12672       +4     
  Branches       1295    1297       +2     
===========================================
  Hits           6387    6387              
- Misses         5827    5831       +4     
  Partials        454     454

Impacted Files	Coverage Δ	Complexity Δ
...k/apollo/openapi/v1/controller/ItemController.java	8.16% <0%> (-0.73%)	1 <0> (ø)

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[coveralls]

Coverage decreased (-0.02%) to 53.985% when pulling 954cdaf on Accelerater:#2823 into 4ce3d70 on ctripcorp:master.