rhel7: Fix rpm install failure.

When trying to install the kernel module rpm built for RHEL7,
the install failed with following conflicts:

  # rpm -i kmod-openvswitch-2.3.1-1.el7.x86_64.rpm
    file /lib/modules/3.10.0-123.8.1.el7.x86_64/modules.devname
    from install of kmod-openvswitch-2.3.1-1.el7.x86_64 conflicts
    with file from package kernel-3.10.0-123.8.1.el7.x86_64

    file /lib/modules/3.10.0-123.8.1.el7.x86_64/modules.softdep
    from install of kmod-openvswitch-2.3.1-1.el7.x86_64 conflicts
    with file from package kernel-3.10.0-123.8.1.el7.x86_64

Similar issue has already been reported and solved here:

    https://bugzilla.redhat.com/show_bug.cgi?id=1003267

This commit applies the solution in the link to ovs.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>