Updating Microsoft Azure Branding

This is a minimal impact set of changes for core branding; I haven't touched the majority of the code and string references to Windows Azure.

- Updated cloud background for inside the disk image
- Updated disk image icon set to have a cloud instead of the old Azure flag logo
- New package installer background graphic using marketing asset (no blue color available currently)
- Updates the primary `azure` ASCII art section to replace 'Windows Azure' with the newer branding
- Updated links on contribution pages

Verified that the build scripts both work and the installed product works.