Remote-execution user configuration (technical debt)

Background

Initial remote-execution implementation (!626 (merged)) only support project level configuration. As discussed on the mailing list, the remote build server one might want to use for a particular build should be a user-level configuration and project level configuration should be a convenient way of defining a default for people working around a project.

Task description

• Add user-level configuration entries.
• Add CLI options for specifying and/or overwriting configuration.
• Change element's logic so that remote execution service to use is determine by merging user and project configurations.