DB connection: prevent src address from binding to a VIP

When a service connects to the database VIP from the node hosting this
VIP, the resulting TCP socket has a src address which is by default
bound to the VIP as well. If the VIP is failed over to another node
while the socket's Send-Q is not empty, TCP keepalive won't engage and
the service will become unavailable for a very long time (by default
more than 10m).

To prevent failover issues, DB connections should have the src address
of their TCP socket bound to the IP of the network interface used for
MySQL traffic. This is achieved by passing a new option to the
database connection URIs. This option is available starting from
PyMySQL 0.7.9-2.

We use a new intermediate variable in hiera to hold the IP to be used
as a source address for all DB connections. All services adapt their
database URI accordingly.

Moreover, a new YAML validation check is added to guarantee that new
services will construct their database URI appropriately.

Change-Id: Ic69de63acbfb992314ea30a3a9b17c0b5341c035
Closes-Bug: #1643487

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