core: Multiple Stocks fail with alpha_vantage component

Home Assistant release with the issue: 0.90.1

Last working Home Assistant release (if known): unknown

Operating environment (Hass.io/Docker/Windows/etc.): hassio running on Raspberry Pi

Component/platform: alpha_vantage

Description of problem: Several issues have been opened regarding the KeyError: ‘Time Series (15min)’, startup error and api limit. I read that we could get a premium API key; however, those cost a minimum of $30 USD per month up to $250 USD per month!! There really needs to be some way to do a circular queue “round-robin” of the stocks in the API call to not exceed the value. I think that most of us are not using these stock prices as a real-time indicator to buy/sell stock… we simply want to have a convenient place to see our stock prices.

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant):

---
platform: alpha_vantage
api_key: !secret alpha_vantage_key
symbols:
  - name: Sprint
    currency: USD
    symbol: S
  - name: Apple
    currency: USD
    symbol: AAPL
  - name: Google
    currency: USD
    symbol: GOOGL

Traceback (if applicable):

Tue Mar 26 2019 09:50:56 GMT-0500 (CDT)

alpha_vantage: Error on device update!
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/homeassistant/helpers/entity_platform.py", line 248, in _async_add_entity
    await entity.async_device_update(warning=False)
  File "/usr/local/lib/python3.7/site-packages/homeassistant/helpers/entity.py", line 379, in async_device_update
    await self.hass.async_add_executor_job(self.update)
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/local/lib/python3.7/site-packages/homeassistant/components/sensor/alpha_vantage.py", line 161, in update
    all_values, _ = self._timeseries.get_intraday(self._symbol)
  File "/usr/local/lib/python3.7/site-packages/alpha_vantage/alphavantage.py", line 178, in _format_wrapper
    data = call_response[data_key]
KeyError: 'Time Series (15min)'

Additional information: Issues #15271 offered a solution to get multiple stocks at the same time thus reducing the number of API calls which would also help this situation.

Without implementing something to regulate the API calls, anyone with more than one stock to monitor cannot use this component.