ovsdb-idl: Test script for Python register_columns function

ovsdb-idl: Test script for Python register_columns function

Add test scripts to exercise the register_columns() function of the
Python IDL. Add ability to specify columns in the "idl" command of
test-ovsdb.py. All columns of all tables are monitored by default.
The new "?" option can be used to monitor specific Table:Column(s).
The table and their columns are listed as a string of the form starting
with "?":
      ?<table-name>:<column-name>,<column-name>,...
  e.g.:
      ?simple:b - Monitor column "b" in table "simple"
  Entries for multiple tables are seperated by "?":
      ?<table-name>:<column-name>,...?<table-name>:<column-name>,...
  e.g.:
      ?simple:b?link1:i,k - Monitor column "b" in table "simple",
                            and column "i", "k" in table "link1"

Signed-off-by: Shad Ansari <shad.ansari@hp.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>