Reinstate default FLAGS.compute_driver for folsom

A previous cherry-picked commit moved the compute_driver flag from
the compute manager to the global flags file, but did not take the
flags default value along.  This breaks existing Folsom installations
that still use connection_type.

Note, this is only proposed to stable/folsom to fix a regression. Not
applicable to master as connection_type is gone in Grizzly.

Fixes bug 1081836.

Change-Id: I135d6adc215c40fb9f46f382dbe22fb38b413ff8

nova/flags.py

@@ -420,6 +420,7 @@ def _get_my_ip():
                      ' during a host restart and apply all at the end of the'
                      ' init phase'),
     cfg.StrOpt('compute_driver',
+               default='nova.virt.connection.get_connection',
                help='Driver to use for controlling virtualization. Options '
                    'include: libvirt.LibvirtDriver, xenapi.XenAPIDriver, '
                    'fake.FakeDriver, baremetal.BareMetalDriver, '