add migration format to datacenter config

add migration format to datacenter config

This adds a new format for configuring cluster wide migration
settings.
Those settings include the migration transfer method, secure
(currently ssh) or insecure (tcp), this deprecates the
migration_unsecure parameter which we only keep for backward
compatibility and map it to the new property.
The mapping of the setting should be unproblematic for the user as
exactly the same semantics happen.
Only the case where both, new and old are set at the same time is
problematic, here warn the user and ignore the old setting.

Further the migration network can be set, this denotes the network
used for sending the migration traffic.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>