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
Expose decom.erase_hardware command via API

Expose an async decom.erase_hardware command in the API. Executing
this command simply calls erase_devices on the loaded HardwareManager
implementation.

Change-Id: I238264a4d5903200ad6a30d82b61804c1e3f5916
Tries to advertise valid default IP

During the first heartbeat, the heartbeater asks the agent to check
its advertised address; if the advertised IP is still the default
(None), the agent tries to replace it with the IP of the first network
interface it finds.  If it fails to find either a network interface or
an IP address, the agent raises an exception.

Change-Id: I6d435d39e99ed0ff5c8b4883b6aa0b356f6cb4ae
Closes-Bug: #1309110
Fix test_run_image

This tests an async function, where exceptions will not bubble up
to the caller. So, we need to check the async result's command_status
attribute in order to make sure the call succeeded.

Change-Id: If8c529943acc794252778280a5fadf2c7860eb0d
Merge "Tries to advertise valid default IP"