Add IloDriver and its IloPower module

Add a new ironic driver for managing HP Proliant Gen8 servers
using iLO4. This commit introduces the power module for
IloDriver.

Implements: blueprint ironic-ilo-power-driver
Change-Id: I8d521f67fb14a6132626782b05cd490cd42ba476
Co-Authors: Anusha Ramineni<anusha.iiitm@gmail.com>
Raise appropriate errors on duplicate Node, Port and Chassis creation

This patch introduces NodeAlreadyExists, ChassisAlreadyExists,
PortAlreadyExists and InstanceAssociated exceptions for
appropriate cases.

Partial-Bug: #1277555

Change-Id: Iec2a7cd09022b6a12afacec9b9b51f8197817ded
Merge "Add IloDriver and its IloPower module"
Add missing docstrings

The ipmitool.VendorPassthru class was missing docstrings for
the methods validate and vendor_passthru. This adds them.

Change-Id: I06d6f32a156da625ec5c47a39f2506d129e1a7b1
Merge "Raise appropriate errors on duplicate Node, Port and Chassis creation"
Merge "Add missing docstrings"