Optimize time during PCI alias parsing process

During the creation of an instance, a list of request is build. One of this requested elements are the PCI devices, both from the API call and from the flavor.

In the PCI request gathering from the flavor, the variable "pci_alias", stored in the Nova config file, is always parsed. This step is only needed once, because the information is static.

This blueprint proposes to parse only once this information and store the value obtained in a global variable in this module.