xenserver: Don't require /etc/xensource/network.conf if not on XenServer.

xenserver: Don't require /etc/xensource/network.conf if not on XenServer.

On XenServer systems, the "network mode" (that is, whether the network
uses OpenvSwitch or the Linux bridge) is stored in
/etc/xensource/network.conf.  On other systems, we can't expect it to be
there.  This changes the init script to tolerate its absence, treating that
as meaning that Open vSwitch should be used (otherwise, why did the admin
install it?).

NETWORK_MODE can always be overridden in /etc/sysconfig/openvswitch anyway.

Signed-off-by: Patrick Mullaney <pm.mullaney@gmail.com>
[changed back to read /etc/xensource/network.conf if present]
Signed-off-by: Ben Pfaff <blp@nicira.com>