code: function createConfigSetString(mapEnvSettings) {
    const settingsStrsArr = map(mapEnvSettings, strFromEnvironmentVarSetting)
    const settingsStr = compact(settingsStrsArr).join(' ')
    // Get project from passed options, falling back to branch name
    const projectKey = getProjectKey(mapEnvSettings)
    return `firebase functions:config:set ${settingsStr} -P ${projectKey}`
  }
docs: Combine all functions config sets from mapEnv settings in
  .firebaserc to a single functions config set command string.
  @param {Object} mapEnvSettings - Settings for mapping environment