Add ignore_absent_pulp_repos flag
    
    For `pulp` source, if a content set is specified but does not exist in
    Pulp instance, an error is returned to the caller and no repofile will
    be created.
    
    When this flag is set, any content set absent in Pulp is ignored. This
    is logged on the server, but the user will not see this.
    
    If all of them are missing, an empty repo file will be produced.
    
    JIRA: COMPOSE-4008