Implement Openflow 1.4 Vacancy Events for OFPT_TABLE_MOD.

Implement Openflow 1.4 Vacancy Events for OFPT_TABLE_MOD.

OpenFlow 1.4 introduces the ability to turn on vacancy events with an
OFPT_TABLE_MOD message specifying OFPTC_VACANCY_EVENTS. This commit adds
support for the new feature in ovs-ofctl mod-table.
As per the openflow specification-1.4, vacancy event adds a mechanism
enabling the controller to get an early warning based on capacity
threshold chosen by the controller.

With this commit, vacancy events can be configured as:
ovs-ofctl -O OpenFlow14 mod-table <bridge> <table> vacancy:<low,high>
<low,high> specify vacancy threshold values in percentage for vacancy_down
and vacancy_up respectively.

To disable vacancy events, following command should be given:
ovs-ofctl -O OpenFlow14 mod-table <bridge> <table> novacancy

Signed-off-by: Saloni Jain <saloni.jain@tcs.com>
Co-authored-by: Shashwat Srivastava <shashwat.srivastava@tcs.com>
Signed-off-by: Shashwat Srivastava <shashwat.srivastava@tcs.com>
Co-authored-by: Sandeep Kumar <sandeep.kumar16@tcs.com>
Signed-off-by: Sandeep Kumar <sandeep.kumar16@tcs.com>
[blp@ovn.org fixed a few typos]
Signed-off-by: Ben Pfaff <blp@ovn.org>