libvirt: avoid 30 second long test in LXC mount setup

The test_create_propagates_exceptions test case in the libvirt
driver test has a 30 second delay while disk setup times out.

  File "nova/virt/libvirt/driver.py", line 4412, in _create_domain_and_network
    disk_info):
  File "/usr/lib64/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "nova/virt/libvirt/driver.py", line 4308, in _lxc_disk_handler
    self._create_domain_setup_lxc(instance, block_device_info, disk_info)
  File "nova/virt/libvirt/driver.py", line 4260, in _create_domain_setup_lxc
    use_cow=use_cow)
  File "nova/virt/disk/api.py", line 386, in setup_container
    dev = img.mount()
  File "nova/virt/disk/api.py", line 306, in mount
    if mounter.do_mount():
  File "nova/virt/disk/mount/api.py", line 218, in do_mount
    status = self.get_dev() and self.map_dev() and self.mnt_dev()
  File "nova/virt/disk/mount/nbd.py", line 120, in get_dev
    return self._get_dev_retry_helper()
  File "nova/virt/disk/mount/api.py", line 121, in _get_dev_retry_helper
    time.sleep(2)

Mock out the _create_domain_setup_lxc & _create_domain_cleanup_lxc
methods to avoid exercising this codepath which is irrelevant to the
goals of the test case.

Resolves-bug: #1369537
Change-Id: Ic860f83ac0b3b7242658640618b223d89c4db253
Merge "libvirt: avoid 30 second long test in LXC mount setup"