Forcibly set libvirt uri in baremetal virtual power driver

To ensure the baremetal virtual power driver is able to control
all the available libvirt domains when connecting to the remote
libvirtd host, we enforce virsh to use the 'qemu:///system' uri.

Enforcing it to 'qemu:///system' should not break existing
deployments while also fixing issues appearing when it is not
set, see tripleo bug 1226310.

Finally, while the uri is an actual config parameter for the Ironic ssh
driver, Nova baremetal is in soft freeze so it seemed more appropriate to
just avoid polluting the config file with deprecated, obscure settings.

Change-Id: I1c956bb9fa6fd365ebeea42d374c800455293e2b
Related-Bug: #1226310
Merge "Forcibly set libvirt uri in baremetal virtual power driver"