ovs-save: Add a helper command to maintain ofport value.

ovs-save: Add a helper command to maintain ofport value.

This patch adds a helper command: save-ofports. The command
will o/p a shell script that will set the 'ofport_request' column
of the interface table with the value of the corresponding ofport.

This command takes a list of bridges as input and will be used by
a future commit to restore the ofport value across force-reload-kmod.

This command is only useful while trying to upgrade from a pre-1.10
branch to 1.10 or a later branch.

Issue #13556
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>