Validate tool settings before it is executed

Implementator of Tool from Spacelift can use custom "setters" to prepare tool's environment or arguments before an execution.

In case this environment is not set properly or it misses, it fails too late or its handling is treated directly in process method which adds additional boilerplate code and it is not good approach in general.

Consider to add validate() method to Tool which would be called by Spacelift upon execute() call.