Demo plugin improvement

[Gujiawei-Edinburgh]

1. get plugin variables from env var; if not found, fall back to system properties, and finally use default values.
2. make demo plugin to reside in a single directory.

[coveralls]

Pull Request Test Coverage Report for Build 12297769460

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 43 unchanged lines in 11 files lost coverage.
• Overall coverage decreased (-0.08%) to 65.541%

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Files with Coverage Reduction	New Missed Lines	%
com/baidu/bifromq/starter/utils/ClusterDomainUtil.java	1	75.0%
com/baidu/bifromq/basekv/store/range/KVRange.java	2	92.65%
com/baidu/bifromq/basekv/store/range/KVRangeWriter.java	2	75.0%
com/baidu/bifromq/basecluster/fd/FailureDetector.java	2	88.16%
com/baidu/bifromq/basekv/store/stats/StatsCollector.java	2	78.95%
com/baidu/bifromq/basekv/client/scheduler/BatchQueryCall.java	2	72.09%
com/baidu/bifromq/dist/trie/NTopicFilterTrieNode.java	3	83.53%
com/baidu/bifromq/basecluster/memberlist/AutoDropper.java	4	82.62%
com/baidu/bifromq/basekv/store/range/KVRangeFSM.java	4	76.93%
com/baidu/bifromq/dist/trie/TopicFilterIterator.java	6	70.99%

Totals
Change from base Build 12270129972:	-0.08%
Covered Lines:	21799
Relevant Lines:	32253

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💛 - Coveralls