Motivation for changes:

There were some conflicts with secrets plugin when jinja was being used in the config (#1578). The name 'secrets' also seemed too limited for what was possible with the plugin.

Detailed changes:

• Renames the 'secrets' plugin to 'variables'
• Changes jinja tag to {? ?} for replacement to avoid conflicts with jinja in the config
• Removes the need for the 'secrets' prefix on every variable

Addressed issues:

• Fixes Secrets plugin breaks jinja default filter #1578

Config usage if relevant (new plugin or updated schema):

• Root level 'secrets' in config will need to be renamed to 'variables'.
• Any use of secrets in the config will need to have the surrounding tags replaced with {? ?} and remove the reference to 'secrets'. e.g.: {{ secrets.username }} becomes {? username ?}