Add support for resource pools

[benjamink]

When multiple resource pools are defined on a vCenter cluster the following error appears:

* Post-processor failed: Failed: default resource pool resolves to multiple instances, please specify

I don't see a way to specify resource pools, though.

[Srini-B-GIT]

I have slimier issue..

$ ./packer build rhel75.json
vsphere-iso output will be in this color.

==> vsphere-iso: Creating VM...
Build 'vsphere-iso' errored: default resource pool resolves to multiple instances, please specify

==> Some builds didn't complete successfully and had errors:
--> vsphere-iso: default resource pool resolves to multiple instances, please specify

==> Builds finished but no artifacts were created.