Add a HardwareManager method to erase devices

Add erase_devices method to the HardwareManager class. By default this
method iterates block devices, and calls a new abstract
erase_block_device method for each device. This patch includes a
simple implementation of erase_block_device on the
GenericHardwareManager which attempts to issue an ATA secure erase on
supported devices.

Change-Id: I81da065395b8785f636f1b0a0d60c9f1c045441e