ovsdb: Add blacklist_tables

ovsdb: Add blacklist_tables

Currently, 'sync-exclude-tables' command line options are simply stored
in a string. Change the implementation to store it in an shash instead
to improve modularity.

One additional benefit of this change is that errors can be detected
and reported to user earlier.  Adde a 'dryrun' option to
set_blacklist_tables() API to make this feature available to the
command line option parsing and unixctl command parsing.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>