ARQ-1767 Validate tool settings before it is executed

[smiklosovic]

@kpiwko

[smiklosovic]

I was thinking about making that validate() method not abstract, it would be just empty method body in Task class so there would not be a need to modify existing tasks after the update of Spacelift version, however, not having it abstract and not explicitly telling a user to do his tool validation simply hides it from him and validation would be optional and not mandatory.