avoid ipvs collector to fail to initialize if sudo does not exist

and update the log msg to be more precise
log error when no conntrack metric was collected
update according to code changes
pythonic
Merge pull request #15 from hvnsweeting/fix-ipvs-initialize

Looks reasonable
update doc
Merge pull request #16 from hvnsweeting/refactor-conntrack-collector

Refactor conntrack collector