Introduce the condition in the processor with the when keyword

[monicasarbu]

By renaming filters with processors ( #1944), more and more arguments will be available under processors, so introducing the condition with when makes it easier to be spotted.

Before:

processors:
 - include_fields:
     equals:
       proc.pid: 3455
     fields: ["proc.cpu", "proc.mem"]

After:

processors:
 - include_fields:
     when:    
        equals:
           proc.pid: 3455
     fields: ["proc.cpu", "proc.mem"]