examples: Add usage of base64encode

[mjtrangoni]

What this PR does / why we need it:
Showcase the usage of base64encode for the user_data field.
I was looking at using it like on Terraform-side and it works without any issue.

How Has This Been Tested?:
I've run this setup on my Nutanix cluster, and it worked as expected.

Special notes for your reviewer:

• No code change, just extend the current examples. Also the user_data content is exactly the same.
• Removed some non-needed spaces on example/source.nutanix.pkr.hcl.
• Added the default disabled variable, image_export as false, so that someone else get to know it quicker. IMHO, this is the purpose of the examples.
• CLA already sent.

Release note:
NONE

[tuxtof]

thanks for your contribution Mario @mjtrangoni

[tuxtof]

HI @mjtrangoni FYI #134

[mjtrangoni]

Interesting. I had no issues with this. Is it something related to one OS?

[tuxtof]

no your example can't work this is pretty strange

a base64encode("scripts/cloud-init/cloud-config-centos.yaml") just return c2NyaXB0cy9jbG91ZC1pbml0L2Nsb3VkLWNvbmZpZy1jZW50b3MueWFtbA==

who is the base64 version of the file path and you get the file path in the user_data file