XenServer hypervisor plugin

Convert XenServer hypervisor configuration in Nova to the new plugin setup.

Change-Id: I8916560ca3f2dae8b8d8bcb60b7aa2eb5984cbcb
PowerVM hypervisor plugin

Convert PowerVM hypervisor configuration in Nova to the new plugin setup.

Change-Id: I3ec700c5eab3fb2e63a4b2717ddbaf9702974c73
Merge branch 'config-local' into dt-test
Merge branch 'hypervisor-powervm' into dt-test
Merge branch 'hypervisor-xenserver' into dt-test