ovs-vsctl: Merge struct vsctl_info into struct vsctl_context.

ovs-vsctl: Merge struct vsctl_info into struct vsctl_context.

To speed up management operations with many ports, we need to preserve the
cache of bridges, ports, and interfaces from one operation to the next.
One necessary step is to push the "struct vsctl_info" that did the caching
up from the individual functions that need it into a more global structure.
This commit does that, merging it into struct vsctl_context.

This commit also modifies do_vsctl(), the top-level control code in
ovs-vsctl, to keep this part of the vsctl_context unchanged from running
one command to the next.

Signed-off-by: Ben Pfaff <blp@nicira.com>