Allow migrating based on hostname instead of IP address of the host

 For live migration the libvirt driver already supports generating the migration URL based on the compute host hostname if so configured. However for the non live move operations the driver always used the IP address of the compute host based on [DEFAULT]my_ip.

Some deployments rely on DNS to abstract the IP address management. In these environments it is beneficial if nova allows connection between compute hosts based on the hostname (or FQDN) of the host instead of trying to configure [DEFAUL]my_ip to an IP address.

This blueprint introduces a new config option [libvirt]migration_inbound_addr that is used to determine the address for incoming move operations (cold migrate, resize, evacuate). This config is defaulted to [DEFAULT]my_ip to keep the configuration backward compatible. However it allows an explicit hostname or FQDN to be specified, or allows to specify '%s' that is then resolved to the hostname of compute host.