Fix detection of deleted networks in DHCP agent.

The DHCP-agent uses an in-memory networks cache to find out which networks must
be deleted and which ones must be updated. In a case of agent restart the networks
cache is empty and it's not possible to cleanup DHCP-processes serving networks
which were deleted while the DHCP-agent was down. The proposed fix fills the networks
cache when the agent starts using a list of networks which have existing config files.

Fixes: bug #1135948
Change-Id: I27758389755cd19bca9fdbeda9cc1a123370f527
Merge "Fix detection of deleted networks in DHCP agent."