add UEFI boot type

[tuxtof]

What this PR does / why we need it:

Allow changing bootype of builder VM to UEFI

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

Fixes #28

How Has This Been Tested?:

source "nutanix" "linux" {
......
  boot_type        = "uefi"
}

Release note:

allow UEFI Boot Type configuration

[github-actions]

✅ None of your dependencies violate policy!

[ktinthecloud]

@tuxtof can you add the boot_type = "uefi" to the example section of the repo? I'm having a hard time trying to figure out how to use it. Thanks!

Error: Unsupported argument

on source.nutanix.pkr.hcl line 98:
(source code not available)

An argument named "boot_type" is not expected here.

==> Wait completed after 2 microseconds

==> Builds finished but no artifacts were created.

[ktinthecloud]

This is working now. Just had to wait for the v0.2.0 release.