Optional `custom.vpcDiscovery`

[rddimon]

Make the custom.vpcDiscovery optional. We can specify a vpc config just for a function which we need.

...
functions:
  example1:
    handler: handler.example

  example2:
    handler: handler.example
    vpcDiscovery:
      vpcName: '<vpc name>
      subnetNames:
        - '<name of subnet>'
      securityGroupNames:
        - '<name of security group>'
...