Add a task worker mechanism to the namespace manager

Fix concurrency issues, only one netns command can be
executed at the same time for one service instance
Introduce a new config parameter to specify the number
of workers available, which means how many service
instance can processed in parallel.

Change-Id: Ic876314ec09f579fc3a534ba8fe3b6c4ee38cd94
Add timeout mechanism to the namespace manager

Change-Id: I8fd539b111bded85c5133c520449bd7d1d188e78
Add more test for the namespace manager

Change-Id: Ib6093cd35a8c5ccd2c58079a48fd86a9de0b49d8
Merge pull request #9 from safchain/service_instance

Service instance