feat: Adds abandonInstances to InstanceGroupManger API

[modeswitch]

Fixes #278

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[modeswitch]

@JustinBeckwith Another patch for you!

[modeswitch]

@stephenplusplus Fixed. I also updated the other methods to use fat arrow functions (deleteInstances, getManagedInstances, and resize).

[stephenplusplus]

Nice, thanks!

[modeswitch]

@JustinBeckwith I think this is ready for kokoro.

[codecov]

Codecov Report

Merging #306 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
+ Coverage   99.14%   99.14%   +<.01%     
==========================================
  Files          20       20              
  Lines        1517     1522       +5     
==========================================
+ Hits         1504     1509       +5     
  Misses         13       13

Impacted Files	Coverage Δ
src/instance-group-manager.js	100% <100%> (ø)	⬆️

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