qcnl: allow users to specify sgx_default_qcnl.conf path

[mythi]

/etc/sgx_default_qcnl.conf is hard-coded in the code but in some cases it could be preferable to be able override that, e.g., with the sample quote verification:

QCNL_CONF_PATH=/my/path/sgx_notdefault.qcnl.conf ./app -quote foo.dat

[llly]

Will provide this feature in 1.22 release.