Better handling of missing drivers

Instead of loading all available drivers, this patch adds add a CONF
option for "enabled_drivers" which explicitly sets the expected drivers.
A failure to load an enabled driver will prevent the conductor service
from starting.

Use a callback hook in stevedore to capture and clean up import exceptions.
Third-party drivers are expected to use importutils to attempt to load
their external requirements, and when unable to, they should raise a
DriverNotFound error to tie into this hook.

Sets the default enabled drivers to:
- pxe_ssh
- pxe_ipmitool

tests/api/v1/test_ports was creating a ConductorManager instance
instead of appropriately mocking the conductor.rpcapi, which led to test
failures when changing how ConductorManager loads drivers. This patch
corrects this by properly mocking the rpcapi within that test class.

Change-Id: I2da37409cea98731a2ca18dfe7298861fde37783
Closes-bug: #1298529
Closes-bug: #1302562