Initialize iptables rules on initialization of MetadataManager

To avoid multiple initialization of iptables rules if there are a few
workers for metadata service, perform iptables configuration in
__init__() of MetadataManager.

Change-Id: I674c04f973318f06cbb98693f0a884c824af8748
Closes-Bug: #1097999
Merge "Initialize iptables rules on initialization of MetadataManager"