Implements ProcessMonitor in the dhcp_agent

Implements ProcessMonitor in the dhcp_agent

The ProcessMonitor class will watch over spawned external processes,
taking the administrator configured action in the case of any
of the external processes die unexpectedly.

It covers both the neutron-ns-metadata-proxy for isolated metadata
and dnsmasq in the dnsmasq driver.

ProcessMonitor has been extended to allow specific pid files
for backwards-compatible dnsmasq pid file location.

Implements: blueprint agent-child-processes-status
Change-Id: I0a4a509b65aadf825799c98dfa48a655b123b403