Enable deployers to specify virtual.instance.plugin in blazar.conf
Enable deployers to specify virtual.instance.plugin in blazar.conf

Partially implements: blueprint new-instance-reservation
Change-Id: I2632f09e9fbbdc7eb9e8e01998fa4043a60b8c9e

openstack/blazar,stackforge/blazar,openstack/blazar,stackforge/blazar,ChameleonCloud/blazar,ChameleonCloud/blazar