[nova] Provide ability to implement alternative live_migration_flag settings

[odyssey4me]

Currently live_migration_flag is not set in the nova.conf template. This leaves the setting to the defaults, which is good.

However, there may be situations where we'd want to override the default behaviour:

1. When you wish to do true live migration, the VIR_MIGRATE_LIVE flag needs to be appended to the defaults.
2. When you wish to do live migration, but you might be using the virtio cache=writethrough setting and therefore live migration won't work (see https://bugs.launchpad.net/nova/+bug/1222682), you'll want to set the VIR_MIGRATE_UNSAFE flag.

I suggest that the attribute to make use of is available, but defaults to nil. The template can then include the configuration option if the attribute has a value that is not nil.

This was originally requested in https://github.com/rcbops/chef-cookbooks/issues/399, but not accepted because the need was a little differently expressed.