r/virtual_machine: Assert timezone using stdlib and TF core

[vancluever]

Reference: #244 (comment)

Right now we are validating this using StringMatch, but a much cleaner way would be to validate using LoadLocation in stdlib, which loads data from the IANA TZ database.

This should be a validation function in TF core: https://github.com/hashicorp/terraform/blob/master/helper/validation/validation.go

[hashibot]

This issue has been open 180 days with no activity. If this issue is reproducible with the latest version of the provider and with Terraform 0.12, please comment. Otherwise this issue will be closed in 30 days.

[ghost]

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