add rolling update instance count

[GwonsooLee]

Add rolling update instance count option in order for users to run rolling update with custom instance count.

Users can use rolling_update_instance_count in stack configuration. If replacement_type is not RollingUpdate, then this configuration will be ignored. If it is and rolling_update_instance_count is null, then 1 will be applied.

[codecov[bot]]

Codecov Report

Merging #126 (883e576) into main (53c9031) will decrease coverage by 0.11%. The diff coverage is 12.50%.

@@           Coverage Diff            @@
##            main    #126      +/-   ##
========================================
- Coverage   5.00%   4.89%   -0.12%     
========================================
  Files         28      28              
  Lines       4632    4598      -34     
========================================
- Hits         232     225       -7     
+ Misses      4389    4361      -28     
- Partials      11      12       +1     

Impacted Files	Coverage Δ
pkg/builder/builder.go	46.53% <0.00%> (+5.25%)	:arrow_up:
pkg/deployer/bluegreen.go	0.00% <ø> (-5.32%)	:arrow_down:
pkg/deployer/canary.go	1.23% <0.00%> (ø)
pkg/deployer/rollingupdate.go	0.00% <0.00%> (ø)
pkg/runner/runner.go	4.43% <0.00%> (ø)
pkg/deployer/deployer.go	1.53% <29.41%> (-0.30%)	:arrow_down:

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[homi-e]

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