Make systemd-modules-load.service file directory configurable

Make systemd-modules-load.service file directory configurable

Installing outside of the prefix is not permissible under Gentoo Prefix.
The package manager will cause the installation process to fail if/when
it sees this. We could handle this by disabling systemd support on
prefix because systemd does not check these paths, but the Gentoo
Council decided that small files such as these should be installed.
That means disabling systemd support on prefix is not an acceptable
workaround. As a consequence, we need some way of control the directory
into which these files are installed.

Making this configurable increases our compliance with the
freedesktop.org specification, which allows these files to be installed
into /etc/modules-load.d:

http://www.freedesktop.org/software/systemd/man/modules-load.d.html

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2641