ofagent (python) does not handle non-existent controllers well

In the Python versions of of-agent (V1.x & V2.x), multiple controllers can be specified when running the container.

If one or more controllers do not exists, then on agent deletion (when an logical device is delete) by the connection manager, an exception may occur.   This results in the agent map/dpid maps not always being cleaned up properly.

When the logical device is created again, it may not be admitted properly and this also may result in ONOS not being notified of its existence.  The Logical Device will show up in the VOLTHA CLI, but often not in the ONOS CLI  (devices command).

I've attached a possible fix for this issue (pre-meter/tech-profiles !!!), but currently do not have time to investigate it further with the 1.x master-tip.  Just wanted to let everyone know.  Hopefully I can get some free time and create a patch set for v1.x and v2.x.  v2.x may not be really needed since ofagent is being written in Golang.