This adds consts for the environment variables that are supported
by the client. These environment variables are unlikely to change,
or at least, unlikely to be removed, but having consts allows for
them to be documented.
I did not change all occurrences of these variables to use the const,
as they're used in various tests, and it's ok to use a fixture for
those, but it's nice to have a const available for (external) consumers
of the client package, and to have their purpose (and caveats)
documented in the code.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
