Support ISO8601 duration format for UpdatePolicy PauseTime

Adds a timeutils module in heat common to handle ISO 8601
duration format. Separating this as a different patch per
request.

Change-Id: I2fe6fccde838f4ddac6b898e4518ffb33e2a76fc
Implements: blueprint as-update-policy
Merge "Support ISO8601 duration format for UpdatePolicy PauseTime"