Spotinst: Ignore volume type case sensitivity to prevent unnecessary updates

[liranp]

This PR ensures the volume type is always in a lowercase format to prevent unnecessary updates:

...
Will modify resources:
  Elastigroup/master-us-west-2a.masters.[...]
    RootVolumeOpts           {"Type":"GP2","Size":64} -> {"Type":"gp2","Size":64}
...

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[rifelpet]

/lgtm

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