Use SIGUSR1 to notify l3 agent of changing prefix file

Use SIGUSR1 to notify l3 agent of changing prefix file

It is common for all OpenStack services to use SIGHUP signal to
reload configuration and restart. This functionality, including
defining signal handler, is defined by oslo.service.

Meanwhile, this is currently not so for l3 agent. PrefixDelegation
class that is instantiated in L3NATAgent overrides handler for SIGHUP,
thus removing handler that was set in oslo.service.

The proposed solution is to use another signal, such as SIGUSR1,
instead of SIGHUP to notify l3 agent that a prefix file was
somehow changed.

Added a functional test for restarting L3 agent using SIGHUP.

Change-Id: I48eb4697a5fad97bcf08cfe1f80921a46d94029d
Closes-Bug: #1511401