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: I06a89f0015c8f188d212995ed0652c5ee2cc1c47
Related-Bug: #1371550