Log ssh commands at trace level

Reduce noise for when we start
serialising tools in cloud-config.
cloudinit: s/AddFile/AddTextFile/, add AddBinaryFile
Merge pull request #573 from axw/cloudinit-addfile-bytes

Change cloudinit.Config.AddFile to take []byte

In preparation for serialising tools in cloud-config for bootstrap
machines.

Also, log ssh commands at trace level

(Reduce noise for when we start serialising tools in cloud-config.)