Add scheduler filters to support uefi boot mode in ironic virt driver

This specification proposes to add new filters to be used with ironic virt driver.
These filters will be used to select the ironic node which supports a given boot_mode - bios or uefi.

1. A filter which uses image property to select the node.
   The required boot mode is given as an image property - boot_mode=uefi|bios. The filter will extract
   this image property and compares it with the ironic node property "supported_boot_modes".

2. A filter which uses a key-value pair present in flavor extra_specs field.
   The required boot mode is given as boot_mode=uefi|bios in flavor extra_specs field. This filter will
   extract the "boot_mode" from flavor extra_specs filed and compares it with the ironic node property
   "supported_boot_modes".