Add support for default targets

Description

Added functionality to all bst commands where applicable to use default elements

Running most commands without a defined element will now do one of 2 things.

1. use a defined "default element" This element is declared in the project.conf
2. If no default element is defined, buildstream will attempt to use all elements in the project

This is so you can run commands against all files in a project, not just those connected to a specific element.

This merge request is related to: #638 and #640 (closed)

This depends on !937 (merged)