with switch-features-mandatory set to true, switch without required features should not be allowed to connect

In the Lithium plugin redesign we now have a configuration to make switch features required:

~$ egrep features distribution-2182/etc/opendaylight/karaf/42-openflowplugin-Li.xml
<switch-features-mandatory>true</switch-features-mandatory>

By default it is false and allows switches (e.g. OVS 2.0.2) that do not support all the
features to still operate as they have in the past with ODL.

If that config is set to true, we are currently still holding a connection with an
unsupported switch. The switch will be shown in operational store as connected and
thus will even show up in the GUI. However, we cannot interact with the switch as
we'd expect (e.g.pushing flows, reading tables). A REST call to config store to get flows
for this connected device will hang for a while and finally give a 500 internal error.

If there is a good reason to retain the connection of this switch without the mandatory
features, then we need to make it clear to the user and be able to handle REST interactions
without hanging. I don't know of a good reason, so my current opinion is that we can
just reject the connection up front.