Fix networks in _PrepareNicModifications()
Fix networks in _PrepareNicModifications()

Passing --net 0:add,ip=5.5.5.5 failed due to a reference
of a non initialized variable (new_net_obj). Reorder the checks
and add some comments to be readable.

Signed-off-by: Dimitris Aragiorgis <4514117c4a99256e19437c7e3407c4ecbf98aec0@grnet.gr>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

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