Improve Swagger generation

[clementdenis]

• Generate operation ids based on api name, version, resource name and method name to avoid conflicts
• Try to make generated swagger files identical over runs by using only order-preserving maps => useful when Swagger spec is generated on build and versioned

[codecov-io]

Codecov Report

Merging #171 into master will decrease coverage by 0.03%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #171      +/-   ##
============================================
- Coverage     80.09%   80.06%   -0.04%     
  Complexity     1715     1715              
============================================
  Files           159      159              
  Lines          5713     5718       +5     
  Branches        754      754              
============================================
+ Hits           4576     4578       +2     
- Misses          854      856       +2     
- Partials        283      284       +1

Impacted Files	Coverage Δ	Complexity Δ
...oogle/api/server/spi/swagger/SwaggerGenerator.java	78.34% <100%> (+0.43%)	60 <3> (+1)	:arrow_up:
.../api/server/spi/config/model/SchemaRepository.java	92.42% <100%> (ø)	39 <0> (+1)	:arrow_up:
...a/com/google/api/server/spi/BackendProperties.java	89.28% <0%> (-10.72%)	11% <0%> (-1%)
...c/main/java/com/google/api/server/spi/EnvUtil.java	70% <0%> (ø)	8% <0%> (ø)	:arrow_down:

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