Move module vars into session class vars
Rename env vars to session vars

There's already a concept of OS environment variables, so
renaming the session variable mapping from environment vars
to session vars makes things a little more clear.
Rename get_variable to get_config_variable

get_variable is aliased so this is s backwards compatible change.
Add support for per session config vars

You can use set_config_variable to set a config var
for a specific session instance.
Add session config vars for metadata retry/timeouts

In addition, num_retries has been mapped to num_attempts, as it's
easier to think about the total number of times vs. the first time
and the number of retries.
Add default param to get_config_variable
Merge branch 'metadata-retry-config' into develop

* metadata-retry-config:
  Add default param to get_config_variable
  Add session config vars for metadata retry/timeouts
  Add support for per session config vars
  Rename get_variable to get_config_variable
  Rename env vars to session vars
  Move module vars into session class vars