Avoid the need for Azure storage emulator when running timer-triggered functions locally

#### What problem would the feature you're requesting solve? Please describe.
The user shouldn't have to install Azure storage emulator to test or debug timer-triggered functions locally.

#### Describe the solution you'd like
The Functions host should detect that, say, the Azure storage connection string is not present and only show a warning that it won't guarantee that a single instance will be called, but it should still execute the function.

#### Additional context
I opened this issue [https://github.com/Azure/azure-functions-core-tools/issues/1592] to track the problem of when the function is run locally, the user has no clue that what is missing is the Azure storage emulator. That issue is now tracking the error message, but it would be great if we didn't really *need* the emulator in the first place, since when running locally it's in theory simpler to only run one instance of the function.

No files or tests are named. Start by tracing how the Functions host handles timer-triggered execution and the Azure storage connection string locally; determine where the emulator requirement is enforced. Done means a missing storage connection produces a warning, the timer-triggered function still runs locally, and the single-instance guarantee is clearly not promised.

Written by the indexing model from the issue text.