Allow capital letters to match valid service name, such as in MyApp

[ttilberg]

Currently if your service name has capital letters, such as in MyApp, it triggers the new ensure_valid_service_name validation error. The error message states alphanumeric characters, but does not allow capital letters.

This change allows capital letters in the service name, such as MyApp.

Fixes #769

[ttilberg]

Alternative to this, if you did in fact intend to disallow capital letters, the error message should be improved. However, I've had an app deployed for some time now called DemandAtlas that has not previously had issues with Traefik.