Configure Proxy Cookie Name from Env Var

[alexbakerdev]

Is your feature request related to a problem? Please describe.
Generally this is just making it possible to set the proxy CookieName via the environment.

One scenario where its useful is having multiple auth environments for a single cookie domain (like staging and production subdomains). E.g.

Cookie Name	Domain
_sso_proxy_staging	staging.example.com
_sso_proxy	example.com

Now the staging proxy can read a valid token from the cookies of the request

Describe the solution you'd like
Make the cookie name configurable via an environment variable.

Describe alternatives you've considered
N/A

[jphines]

Closed by #198