config: allow specific openssl binary to be used

config: allow specific openssl binary to be used

Provide a $Girocco::Config::openssl_bin setting that allows
a specific `openssl` binary to be specified.  If left `undef`,
the default continues to be the `openssl` found in $PATH.

Additionally, arrange for CACreateCert to use the same
openssl binary when creating certificates by prepending the
"$basedir/bin" directory (which now contains a symbolic link
to the desired copy of `openssl`) to $PATH before running
CACreateCert.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>