EuiSwitch doesn't announce its label

[cjcenizal]

This might just be a bug with the example page ("Form controls"), but VO just announced "Checked, checkbox" when tabbing to the EuiSwitch example. It should also announce the label associated with the switch.

[cchaos]

It needs and id prop that we're not automatically adding inside the component if it's not passed down. Otherwise there's nothing to hook up the htmlFor attribute to to link the label and checkbox.

[cjcenizal]

@cchaos I just took a look at the implementation... it looks like EuiSwitch automatically generates an id if one isn't provided; the problem is that we're just not supplying that to the label. Should be a simple fix. I'll submit a PR now.