Create vApp VM Security Tag Tests

[adezxc]

This PR introduces security_tags field tests of vcd_vapp_vm and vcd_vm resources that got introduced in #1006

[Didainius]

A few initial comments.

• The vcd_security_tag is not necessary to create tag directly on vcd_vapp_vm or vcd_vm. We need to test at least such cases:

  • Create a security tag only by security_tags field on vcd_vapp_vm and vcd_vm (to check if it worked - we could either plug in a govcd function, or add data sources of these resources and check their values as we need to validate data sources anyway)
  • Improve TestAccVcdSecurityTag by checking if vcd_security_tag field gets populated on a vcd_vapp_vm resource that is being used in the test
  • Check a scenario when you create a security tag on in vcd_vapp_vm and what happens when you delete that VM (is the tag removed completely or left as orphan without any VMs)
  • Check that once you create a tag using security_tags on a VM and then remove that field from vm definition - there is still a tag on that VM (this will require using govcd function, probably testAccCheckSecurityTagOnVMCreated can be reused)

• We also need to add vcd.ResourceSchema-vcd_vapp_vm.tf and vcd.ResourceSchema-vcd_vm.tf to skip-upgrade-tests.txt because their schema has changed (security_tags field added)

[Didainius]

LGTM, thanks