Adding /usr/bin to exec path so /usr/sbin/cacertdir_rehash works

On RHEL 7, authconfig attempts to run /usr/sbin/cacertdir_rehash, which assumes that /usr/bin is in the path. If /usr/bin isn't in the path, the cacertdir_rehash fails silently, which causes the certs not to be rehashed.

This change has been tested on RHEL 6 and RHEL 7 with no ill effects.