rgw: fix opslog operation field as per Amazon s3

[mikulely]

rebased #16984 with master.

According to Amazon s3[1], current Operation field are using
http method, which should be operation name.

[1] http://docs.aws.amazon.com/AmazonS3/latest/dev/LogFormat.html

Fixes: http://tracker.ceph.com/issues/20978

Checklist

• References tracker ticket
• Updates documentation if necessary
• Includes tests for new functionality or reproducer for bug

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[mikulely]

@cbodley would you mind taking a look?

[mikulely]

hi @cbodley mind to merge this?

[stale]

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