Ensure pid file is removed when metadata ns daemon receives SIGTERM

These files from the metadata namespace proxy are not being removed
because delete_pid() is registered with atexit. This means it only runs
when a process exits normally and won't run when a process receives a
signal.

This patch registers a signal handler for SIGTERM that calls exit()
to make the process exit normally so delete_pid() gets called.

Fixes bug: 1223250

Change-Id: I6309802e2109359560ccc084559ec8e4d310cce2
Support for NVP advanced service router

When creating an LR:
    - deploy an Edge asynchronously
    - create a L2 switch for connecting LR and Edge
    - attach a router port to the L2 switch.
    - assign ip address 169.254.2.1/28 and nexthop 169.254.2.3 to LR
When set external gateway:
    - configure Edge interface and default gateway
    - Add static routes to Edge for all logic networks attached to LR via nexthop 169.254.2.1
    - configure SNAT rules for all logic networks attached to LR
When add router interface:
    - Add static route/SNAT rule for the network attached to LR
When associate floating IP address:
    - configure DNAT rule for the floating ip and the port

Tests being done:
    - Verified Edge is deployed asynchronously and LR is attached to the internal created L2 switch
    - Manually attach Edge's vNic to the L2 switch and Edge is able to ping 169.254.2.1
    - Verified router-delete deletes Edge asynchronously and remove the internal L2 switch
    - Verified SNAT/DNAT/static-routes rules are configured on Edge in correct order
    - Verified external vnic ip address/netmask and default gateway is configured

Implements: blueprint nvp-service-router
Change-Id: If9eff53df4d65cf4e318dedbfaafc742f6c6ab7f
Merge "Ensure pid file is removed when metadata ns daemon receives SIGTERM"
Merge "Support for NVP advanced service router"