Fix skipping templates for Drone 1.x

[tonglil]

Drone 1.x (tested in 1.7) drops environment variables whose value is an empty string "", eg PLUGIN_VAR="" is removed from the environment.

In order to remove dependency on how Drone exposes environment variables, this stops using the "" sentinel value to determine skipping processing a template or secret_template file.

Instead explicit configuration is required to skip processing those files if they exist in the working directory:

• skip_template: bool
• skip_secret_template: bool

[mlclmj]

this is awesome--thanks!

[Emmawaterman]

Thank you so much!