Make rescheduling of l3 agent more resilient

In order to make l3 agent restart more resilient
and idempotent do:

1) delete agent after resource is stopped and
all the stuff is cleaned up

2) reschedule not only dead routers/networks but also
orphaned routers/networks

3) enclose get_pid_list_for_ns_list function argument
in cleanup function into double quoutes
or it will use only the first argument
and will not work with several networks

Change-Id: I90338ff2df2703e121a6075ad1e60a442c88591f
Related-Bug: #1371550
Related-Bug: #1375817
Merge "Make rescheduling of l3 agent more resilient" into stable/5.1