Accept localtime xor utc

[ganmacs]

Which issue(s) this PR fixes:
None

What this PR does / why we need it:

I think localtime = true && utc =false and locatime= false && utc = true should be valid.

In the current implementation, stdout format plugin raises the error like below with utc true in format section.

2019-12-10 15:06:02 +0900 [error]: config error file="example/debug/nginx.conf" error_class=Fluent::ConfigError error="both of utc and localtime are specified, use only one of them"

example conf

<match *>
  @type stdout

  <format>
    @type stdout
    utc true
  </format>
</match>

This is because of calling TimeParameters#configure twice.
but it seems to be inevitable since the stdout format plugin has sub_formater object internally.

Docs Changes:

No need

Release Note:

same as the title

[repeatedly]

Looks good.