add available cluster support

[xujianhai666]

What this PR does:

add available cluster support

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #155 into master will increase coverage by 0.06%.
The diff coverage is 80.95%.

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
+ Coverage   67.89%   67.95%   +0.06%     
==========================================
  Files          75       77       +2     
  Lines        4495     4516      +21     
==========================================
+ Hits         3052     3069      +17     
- Misses       1130     1132       +2     
- Partials      313      315       +2

Impacted Files	Coverage Δ
cluster/cluster_impl/available_cluster.go	100% <100%> (ø)
cluster/cluster_impl/available_cluster_invoker.go	73.33% <73.33%> (ø)

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[hxmhlt]

LGTM