Allow overriding username with dynamic config

[davejlong]

Using the dynamic config API, we should enable overriding the VictorOps username value with a dynamic config.

Using a user layer, the VictorOps can be overridden from the default (Cog username) on a per user basis, as needed.

[davejlong]

This is a great task to introduce beginners to some Cog concepts. The only change to the code is at lib/cog_cmd/victor_ops/helpers.rb#L33. Instead of just using the Cog Username key, the code has to look for something like a VICTOROPS_USERNAME key in the ENV object.

The dynamic config would be something like:

VICTOROPS_USERNAME: dlong

Which would be deployed with cogctl bundle create --layer user/davejlong victorops ./victorops_davejlong.yaml