Introduce os:getenv/2

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Start using os:getenv/2 fun

See #535

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
fix missing include
Merge branch 'lemenkov/use_os_getenv_2'

* lemenkov/use_os_getenv_2:
  fix missing include
  Start using os:getenv/2 fun
  Introduce os:getenv/2