nvp:log only in rm router iface if port not found

The code currently raises when it's not able to find the port to be
deleted on the backend. This might happen, for instance, when the
port is manually removed or, in some cases, if port creation failed
in the first place.

There's little sense in raising when a resource to be removed is not
found; therefore this patch replaces the raise statement with a log
statement.

Bug 1240720

Change-Id: Ifa756b07ef41346a44325341c8e0e8994dc64dde
(cherry picked from commit 477dac0373b5e84c71365d7a71b9cc41aafcb491)
Merge "nvp:log only in rm router iface if port not found" into stable/havana