Exclude .pkrvars.hcl files

* Used description for .tfvars in Terraform.gitignore as a template

Packer.gitignore

 # Crash log
 crash.log
 
+# https://www.packer.io/guides/hcl/variables
+# Exclude all .pkrvars.hcl files, which are likely to contain sentitive data, 
+# such as password, private keys, and other secrets. These should not be part of 
+# version control as they are data points which are potentially sensitive and 
+# subject to change depending on the environment.
+#
+*.pkrvars.hcl
+
 # For built boxes
 *.box