SHDP-309 Fix minicluster for 2.3.0

- This adds a little hack to work around
  core-site.xml added in jar containing
  MiniYarnCluster.
- We register our own shdp-site.xml to override
  default core-site.xml in tests jar.
SHDP-311 Fix yarn def classpath for 2.3.0

- 2.3.0 removed classpath setting from yarn-default.xml
  effectively breaking tests because we assumed it was there.
- Now we check if value is empty and fall back to one
  defined in YarnConfiguration.
SHDP-308 Fix test usage for minicluster

- New functionality in 2.3.0 doesn't work
  properly with multiple running clusters
  where node managers are short circuited
  to talk to resource managers.
- This fix dirties context in our own tests
  to get fresh cluster for every test class so
  that old one is stopped.