KYLIN-4412 Show cluster name in email title

[zhoukangcn]

see: https://issues.apache.org/jira/browse/KYLIN-4412

[coveralls]

Pull Request Test Coverage Report for Build 5727

• 0 of 5 (0.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.007%) to 27.511%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
engine-mr/src/main/java/org/apache/kylin/engine/mr/CubingJob.java	0	5	0.0%

Files with Coverage Reduction	New Missed Lines	%
core-dictionary/src/main/java/org/apache/kylin/dict/lookup/cache/RocksDBLookupTable.java	1	81.08%
stream-core/src/main/java/org/apache/kylin/stream/core/storage/columnar/ColumnarStoreCache.java	1	56.82%
core-cube/src/main/java/org/apache/kylin/cube/cuboid/TreeCuboidScheduler.java	2	68.46%
core-cube/src/main/java/org/apache/kylin/cube/inmemcubing/MemDiskStore.java	5	77.81%

Totals
Change from base Build 5720:	-0.007%
Covered Lines:	24295
Relevant Lines:	88309

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💛 - Coveralls

[shaofengshi]

+1 LGTM