Add DiskIOPS and EncryptEBS options for create cluster provider settings

[reybard]

I am using this lib to help write a DB automation tool and found that we needed a couple of options that were not natively exposed. I added them in my fork and figured upstream could benefit.

[codecov-io]

Codecov Report

Merging #6 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #6   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines         305    305           
=====================================
  Hits          305    305

Impacted Files	Coverage Δ
mongodbatlas/clusters.go	100% <ø> (ø)	:arrow_up:

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[akshaykarle]

Thanks @jescochu :)