Add previewReplicaCount to BG

[dthomson25]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@4b59abf). Click here to learn what that means.
The diff coverage is 82.5%.

@@            Coverage Diff            @@
##             master      #64   +/-   ##
=========================================
  Coverage          ?   78.01%           
=========================================
  Files             ?       15           
  Lines             ?     1669           
  Branches          ?        0           
=========================================
  Hits              ?     1302           
  Misses            ?      276           
  Partials          ?       91

Impacted Files	Coverage Δ
utils/replicaset/replicaset.go	94.26% <100%> (ø)
utils/replicaset/bluegreen.go	100% <100%> (ø)
controller/sync.go	74.24% <100%> (ø)
controller/bluegreen.go	78.12% <46.15%> (ø)

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[alexmt]

LGTM !