Add ManagementInterface

Add a new driver ManagementInterface for management related actions,
the new interface contains the following methods:

* validate() - To validate driver-specific management information. (E.g
  if the node has the right credentials already set)

* set_boot_device() - To set the boot device for a node.

* get_boot_device() - To get the current boot device of a node.

* get_supported_boot_devices() - To get a list of the supported boot
  devices by that driver.

A new file common/boot_devices.py which contains the standardized names
for the acceptable boot devices, the names are based on the IPMITool
names (the default power driver). The idea is that clients won't need to
know the specific boot device names for each driver, drivers which have
different name options than the IPMITool have to translate it internally.

As part of the work to add tests to the FakeManagement interface, other
tests in test_fake.py were modified in order to remove the DB calls.

blueprint: promote-set-boot-device
Change-Id: If2e8074f2e0fbcf962d224024347a0319ac174cf
Merge "Add ManagementInterface"