Code reliability improvements

[x] The method are not ordered in the most obvious way in the vm resource file - from the usability perspective.
[x] The logging level for spewing the object contents needs to be reduced to DEBUG level. In addition another simplified log entry for INFO level should be added.
[x] Remove global vm disk and vm network name structures from config. Replace with overloaded method. https://github.com/terraform-providers/terraform-provider-skytap/pull/13#discussion_r244756834
[x] Remove unnecessary nil check: https://github.com/terraform-providers/terraform-provider-skytap/pull/10#discussion_r244724394
[x] Remove unnecessary nil check: https://github.com/terraform-providers/terraform-provider-skytap/pull/10#discussion_r244717487
[x] Remove test facilitation code for production code: https://github.com/terraform-providers/terraform-provider-skytap/pull/10#discussion_r244716888
[x] move all tests in resource_skytap_vm_hardware_test.go to resource_skytap_vm_test.go and delete: https://github.com/terraform-providers/terraform-provider-skytap/pull/13#discussion_r244770102
[x] rename adds and ids: https://github.com/terraform-providers/terraform-provider-skytap/pull/13#discussion_r244772944
[x] ~~move conditional outside:~~ invalid https://github.com/terraform-providers/terraform-provider-skytap/pull/13#discussion_r244778921
[x] VM Create test is multistep that perform a modify, we need to divide this test in at least 4. test scenarios. https://github.com/terraform-providers/terraform-provider-skytap/pull/14#issuecomment-450919159 i) create vm ii) modify vm name iii) modify vm cpu iv) modify vm memory
[x] Tidy this line: https://github.com/terraform-providers/terraform-provider-skytap/pull/16#discussion_r244801993
[x] Improve this local variable name: https://github.com/terraform-providers/terraform-provider-skytap/pull/16#discussion_r244802498
[x] Check whether this validation is necessary: https://github.com/terraform-providers/terraform-provider-skytap/pull/16#discussion_r244801522
[x] Add a 422 retry when creating VM.

skytap / terraform-provider-skytap

Code reliability improvements #18