cmdlib: Factorize network conflict detection

LUNetworkConnect and LUNetworkDisconnect had very similar code to detect
conflicts between instance's network interfaces and networks. This code
factorizes the common part and does some cleanup:

- Remove single-letter variable for lambda (“l”)
- Don't repeat instance name in warning output
- Fixed error messages to match style guide

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>