Update name of user agent for collectd to match design doc.
Update name of user agent for collectd to match design doc.

See design doc: http://go/ops-agent-design-m1#heading=h.1v0bnm8mwjjr

Change-Id: I50d471f0c24f73d55db226bfafc6486ce6fed5ce

GoogleCloudPlatform/ops-agent,GoogleCloudPlatform/ops-agent