[explore] DatasourceControl to pick datasource in modal (#3210)

* [explore] DatasourceControl to pick datasource in modal

Makes it easier to change datasource, also makes it such that the list
of all datasources doesn't need to be loaded upfront.

* Adding more metadata

superset/assets/spec/javascripts/explore/components/DatasourceControl_spec.jsx

@@ -0,0 +1,32 @@
+import React from 'react';
+import sinon from 'sinon';
+import { expect } from 'chai';
+import { describe, it, beforeEach } from 'mocha';
+import { shallow } from 'enzyme';
+import { Modal } from 'react-bootstrap';
+import DatasourceControl from '../../../../javascripts/explore/components/controls/DatasourceControl';
+
+const defaultProps = {
+  name: 'datasource',
+  label: 'Datasource',
+  value: '1__table',
+  datasource: {
+    name: 'birth_names',
+    type: 'table',
+    uid: '1__table',
+    id: 1,
+  },
+  onChange: sinon.spy(),
+};
+
+describe('DatasourceControl', () => {
+  let wrapper;
+
+  beforeEach(() => {
+    wrapper = shallow(<DatasourceControl {...defaultProps} />);
+  });
+
+  it('renders a Modal', () => {
+    expect(wrapper.find(Modal)).to.have.lengthOf(1);
+  });
+});