Rename and refactor ExecutorProvider related functions

What does this MR do?

Making code less confusing:

• common.GetExecutor() returns ExecutorProvider so renaming to common.GetExecutorProvider()
• same for common.GetExecutors() -> common.GetExecutorNames() and RegisterExecutor() -> RegisterExecutorProvider()
• executors variable contains a map of executor name to executorProvider, so naming executorProviders
• removing unnecessary nil checks when iterating slices
• adding godoc in some places

Why was this MR needed?

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Added tests for this feature/bug
• In case of conflicts with master - branch was rebased

What are the relevant issue numbers?