Document how to update dependencies for compute images

[rahalan]

If users use a different test environment, gallery images may not be found, as storage account name is generated automatically. That needs to be updated manually.

[AlexanderSehr]

@eriqua, is this issue still valid with the latest code base?

[eriqua]

This has been solved by dependency pipeline update taking care of generating and backing up the vhd needed by the computer images module