Fix unittest failures

failed test case:
cinder.tests.test_nfs.NfsDriverTestCase.test_find_share
cinder.tests.test_nfs.NfsDriverTestCase.test_find_share_should_throw_error_if_there_is_no_enough_place

i think mok can't set temporary value (self.TEST_NFS_EXPORT2).
so modified only using self.TEST_NFS_EXPORT1 for test.