Added support for splunk.secret to be passed as a variable

[chrisbalmer]

This is a PR to resolve issue #50

Added a new task to setup the splunk.secret file as a template if the splunk_secret_text variable is set. I also included an example script to generate a new secret to store in a secret vault using the Splunk UF docker image.

[mason-splunk]

Almost there, but is defined in the when only checks if the variable is actually defined anywhere with a value. Since we are setting a default value with the string value "undefined" in defaults/main.yml, this will always evaluate to true with your current conditionals.