add ubuntu example

[tuxtof]

What this PR does / why we need it:

add ubuntu example

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 #

How Has This Been Tested?:

packer build -only nutanix.ubuntu .

nutanix.ubuntu: output will be in this color.

==> nutanix.ubuntu: Creating Packer Builder virtual machine...
    nutanix.ubuntu: Virtual machine Packer-S8XATSIO created
    nutanix.ubuntu: Found IP for virtual machine: 10.83.7.77
==> nutanix.ubuntu: Using SSH communicator to connect: 10.83.7.77
==> nutanix.ubuntu: Waiting for SSH to become available...
==> nutanix.ubuntu: Connected to SSH!
==> nutanix.ubuntu: Provisioning with shell script: /var/folders/dq/y4w54k297qlcp5xwt_vss2kc0000gn/T/packer-shell3228719099
==> nutanix.ubuntu: Gracefully halting virtual machine...
==> nutanix.ubuntu: Creating image from virtual machine Packer-S8XATSIO...
    nutanix.ubuntu: Found disk to copy: SCSI:0
    nutanix.ubuntu: Successfully created image: ubuntu-packer-image (c2e84222-453b-4a13-bb2e-83198c3354df)
==> nutanix.ubuntu: Deleting virtual machine...
    nutanix.ubuntu: Virtual machine successfully deleted
Build 'nutanix.ubuntu' finished after 1 minute 20 seconds.

Release note:

add ubuntu example

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