cmdlib: Factorize network conflict detection
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 <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

yiannist/ganeti,ganeti/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,bitemyapp/ganeti,bitemyapp/ganeti,andir/ganeti,ganeti/ganeti,leshchevds/ganeti,leshchevds/ganeti,mbakke/ganeti,grnet/snf-ganeti,dimara/ganeti,onponomarev/ganeti,onponomarev/ganeti,apyrgio/ganeti,andir/ganeti,ganeti/ganeti,leshchevds/ganeti,yiannist/ganeti,mbakke/ganeti,dimara/ganeti,mbakke/ganeti,grnet/snf-ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti,andir/ganeti