Implement support for allowing cpu and memory updates to the vm resource

[ddelnano]

This addresses #111 and #40

Todo

• Decide the final scope of this change
  • Should the other missing vm.set attributes be supported?
  • Should the breaking change to support the necessary cpu and memory toggles be made?
  • The breaking changes and full support of the vm.set parameters will be done in a later PR
• Add test that ensures that VM is not shutdown when cpu change is within cpu max
• Add test that ensure that VM is not shutdown when memory change is within memory limits
• Clean up the mocking code needed to validate that online cpu and memory changes work as expected
• Create github issue to track supporting the other vm.set parameters in the vm resource update (Support missing parameters in vm.set for Vm updates #145)
• make testacc and make testclient pass
  • make testclient passes
  • make acctest passes

[ddelnano]

This will be released in v0.19.0