Configurable Instance Boot Source list for launch instance

Summary
=======
Allow the operator to mark boot sources as available or disabled

Motivation
========
The deployer of horizon might want to limit the boot source options to one or several items while disabling others.

Description
=========
Allow the operator to mark boot sources as available or disabled in the local_settings.py by using a variable OPENSTACK_SOURCE_BOOT similar to OPENSTACK_NEUTRON_NETWORK options.
By default, all sources should be enabled unless explicity disabled.

OPENSTACK_SOURCE_BOOT = {
    'enable_image': True,
    'enable_snapshot': True,
    'enable_volume': True,
}

If all source boot options are disabled then no source boot options should appear and a message stating that there is no boot sources enabled should be displayed.

UX
===
The boot source list would offer one or more items.
If all source boot options are disabled then no source boot options should appear and a message stating that there is no boot sources enabled should be displayed.

Testing
======
Testing needs to be done to make sure that the options are reflected and that the limits do not break actual code

Outside Dependencies
==================
No outside dependencies

Requirements Update Required
========================
No requirements

Doc Impact
=========
Should mention on the release notes that its possible to configure and restrict the boot source.