Add an option to overwrite existed lambda functions

If a function already exists in AWS lambda, following error will be raised when running ```chalice deploy```

```
ERROR - While sending your chalice handler code to Lambda to create function
"somefunction", received the following error:

An error occurred (ResourceConflictException) when calling the CreateFunction
operation: Function already exist: somefunction
```

Will it be better if an option is added to enable overwriting existed lambda functions? This is because it is very common to have existed lambda function due to deployment failure or deleting ```deployed.json``` accidentally.

Start with the `chalice deploy` path and the Lambda CreateFunction failure shown in the issue. Determine how an option should handle functions that already exist, including deployments after a failed deploy or a deleted `deployed.json`; done means the option allows the existing Lambda function to be overwritten without the ResourceConflictException.

Written by the indexing model from the issue text.