Revise feat ignore_if_prev_successes to copy plugin

[cosmo0920]

Which issue(s) this PR fixes:
Revised #3190

In this revised version, I added a testcase fix for #3190 (comment).

What this PR does / why we need it:

According to #3130:

This PR makes a change in the Copy Output plugin so you can inform the fluentd to ignore said copy if the previous output succeeded.

This enables having multiple nested outputs as backup of each other by doing:

   <store ignore_error>
     @type test
     name c0
   </store>
   <store ignore_if_prev_success ignore_error>
     @type test
     name c1
   </store>
   <store ignore_if_prev_success>
     @type test
     name c2
   </store>

Docs Changes:

Adding the defined ignore_if_prev_successes attribute to the tag makes the copy plugin ignore said store if the previous store succeed in pushing the output.

Release Note:

Same as title.

[kenhys]

There are some requests to make it user-friendly.

[kenhys]

LGTM