Consistent UX for fetching and pulling

Background

See https://mail.gnome.org/archives/buildstream-list/2018-December/msg00000.html and its follow-ups for details on the discussion.

BuildStream should provide a consistent UX for pulling (or fetching, in case of sources) sources, artifacts, build trees, junctions etc. For all such things, we should provide the following:

• Options for both --pull and --no-pull case (or, --fetch and --no-fetch in case of sources) should be added to the top-level bst command.
• It should be possible for users to configure the default behavior, not just via CLI, but also using config files.
• So long as it is non-intrusive, BuildStream should default for fetching/pulling things that are needed.
• No matter what is the default provided by BuildStream, it should be possible to override it. The overrides should work in the following order (highest priority item listed first):
  1. CLI flag
  2. User config
  3. BuildStream default

Task description

• Add --fetch/--no-fetch as top-level options, with support for user config, and remove similar options from existing commands.
• Add --fetch-junctions/--no-fetch-junctions as top-level options, with support for user config.
• Add --pull/--no-pull as top-level options, with support for user config, and remove similar options from existing commands.
• Add --no-pull-buildtree as top-level option (--pull-buildtree already exists), with support for user config.

Acceptance Criteria

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