Re-enable the dependencies of the spec file

RHEL 6.5 and CentOS 6.5 can satify the required dependencies by adding
the OpenStack related RPM packages. So I'm assuming we can just
re-enable the commented "Requires" section in the spec file.

This resolves [MN1550](https://midobugs.atlassian.net/browse/MN-1550).

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>