Calculate partition power for swift

Calculage partition power according to
https://answers.launchpad.net/swift/+question/211929

power = int(log2(devnumber*100)) + resize_factor

resize_factor can be set in $::fuel_settings['swift'] hash.
default value: 2 (2048 partitions in standard setup)

Change-Id: Icbc518b05f8170b8656b6aad701bf3cece89f74f
Related-bug: #1261659