openstack安装nova控制端同步数据库出现这个报错，麻烦大佬给看看

Traceback (most recent call last): File "/usr/bin/nova-manage", line 6, in from nova.cmd.manage import main File "/usr/lib/python2.7/site-packages/nova/cmd/manage.py", line 73, in from nova.virt import ironic File "/usr/lib/python2.7/site-packages/nova/virt/ironic/init.py", line 16, in from nova.virt.ironic import driver File "/usr/lib/python2.7/site-packages/nova/virt/ironic/driver.py", line 52, in from nova.virt import configdrive File "/usr/lib/python2.7/site-packages/nova/virt/configdrive.py", line 29, in import nova.privsep.fs File "/usr/lib/python2.7/site-packages/nova/privsep/init.py", line 18, in from oslo_privsep import capabilities File "/usr/lib/python2.7/site-packages/oslo_privsep/capabilities.py", line 110, in ffi = cffi.FFI() File "/usr/lib64/python2.7/site-packages/cffi/api.py", line 63, in init from . import cparser File "/usr/lib64/python2.7/site-packages/cffi/cparser.py", line 7, in import pycparser File "/usr/lib/python2.7/site-packages/pycparser/init.py", line 14, in from .c_parser import CParser File "/usr/lib/python2.7/site-packages/pycparser/c_parser.py", line 11, in from ply import yacc ImportError: No module named ply