Specify server list in environment variable

I am attempting to override the server list by the environment variable. I have attempted to set the environment value as `['host1:port','host2:port','host3:port']`. I have also tried variations of this by replacing comma with space, single quotes with double etc. Every time I get `panic: Cluster 'kf_server' has one or more improperly formatted servers (must be host:port)`.

What is the right way to specify a server list in the environment for burrow?

No file, test, or entry point is named in the issue. Start by locating Burrow's environment-variable configuration documentation and the server-list parsing reference; done means documenting the accepted syntax for specifying multiple servers and confirming it avoids the reported formatting panic.

Written by the indexing model from the issue text.