Enable Python Eureka client to accept an application name via environment variable

[sriramkrishnan]

The current Eureka test client uses a keyword arg with appName set to "genie" by default.

Provide a backwards-compatible override mechanism to fetch an alternative application name registered with Eureka, which would take lower priority than the keyword argument itself.

Behavior should be as follows:

• If appName is set, use it
• If appName is not set, then fetch it from environment variable NETFLIX_APP
• If it is still not set, default it to "genie"

[sriramkrishnan]

Fixed.