Bug 8978 - Network deletion issue.

Bug 8978 - Network deletion issue.

Elaninstance after deletion was clearing l2gateway connection
configuration , but was not deleting l2gatewayConnection object itself
from neutron config tree. This leads to reconfiguration of
l2gatewayconnection upon reboot / device reconnect also.
Fixed the above case.
Added way to cancel logical switch deletion , so that if the same
network is reconfgured while deletion is going on , it can be cancelled.
Calling logical switch delete cancel when associating the same network
to l2gw.

Change-Id: I530c4b87becf3043eb8c4ba03226447731b2db0b
Signed-off-by: eaksahu <a.k.sahu@ericsson.com>