ovs-ctl.in: Increase the limit on file descriptors.

ovs-ctl.in: Increase the limit on file descriptors.

Testing shows that creation of 5000 internal ports and using it
to do some meaningful tasks works fine on a 12 cpu hardware.
Since a single port needs one file descriptor and a bridge
needs 3 file descriptors, we will have to increase the file
descriptor limit to a higher number from the current limit of 5000.
7500 feels like a decent increase with enough room for further
scale testing.

Bug #18383.
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>