Use readable time stamps on log messages.

On long running tests, the time might wrap:

[LOG] 72.15087 JUJU state: opening state; zookeeper addresses: ["ec2-23-20-157-185.compute-1.amazonaws.com:2181"]
[LOG] 78.46405 JUJU state: waiting for state to be initialized
destroy env
[LOG] 13.73135 JUJU environs/ec2: destroying environment "sample-6326978ae3647455"

This CL makes the time stamp format as a duration since
the start of the test in the format agreed before:

[LOG] 0:00.568 DEBUG juju.environs.tools reading v1.* tools
[LOG] 0:00.569 INFO juju environs/testing: uploading FAKE tools 1.17.3-trusty-amd64
[LOG] 0:00.570 INFO juju environs/testing: uploading FAKE tools 1.17.3-precise-amd64
[LOG] 0:00.571 DEBUG juju.environs.tools no architecture specified when finding tools, looking for any

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https://codereview.appspot.com/5874049