Adds getManagedInstances and deleteInstances to InstanceGroupManger API

[modeswitch]

Partially addresses #103.

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

[modeswitch]

@JustinBeckwith One more for you!

[modeswitch]

This is ready for kokoro.

[codecov]

Codecov Report

Merging #265 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   99.13%   99.15%   +0.01%     
==========================================
  Files          20       20              
  Lines        1509     1537      +28     
==========================================
+ Hits         1496     1524      +28     
  Misses         13       13

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

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[modeswitch]

Thanks @JustinBeckwith!

[modeswitch]

I think @stephenplusplus needs to approve as well.

[stephenplusplus]

Thank you, @modeswitch!