Add environment-variable substitution to config options

Add environment-variable substitution to config options

Strings of the form ${VARNAME} in config option strings are substituted with
the value of the environment variable VARNAME.  Only the right hand side of
an option assignment undergoes substitution.  If VARNAME is empty or
undefined, the empty string is substituted.

Signed-off-by: Aaron Carroll <aaronc@gelato.unsw.edu.au>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>