Move Unix domain socket helpers to a common place

Move Unix domain socket helpers to a common place

As part of the all consuming report-ha-router-master, a new
per router neutron-keepalived-state-change daemon will alert
the L3 agent on every keepalived state change. Since it will
use the Unix domain socket helpers, and they're currently
located in metadata related places, this patch moves them
to a common location.

Also, the UnixDomainHTTPConnection connection string
may now be overridden.

Partially-Implements: blueprint report-ha-router-master
Change-Id: Ib2cde90059f4e089064b2def2838e9bcf9af30de