Do not clean docs directory
Do not clean docs directory

The docs/source has been introduced by change-id:
Ide5f7d980a294c7a9f8a3decaed0939f1c239934, but clean.sh try to clean
whole docs direcotry, which is not right.

Meanwhile, there are directories in FILES_TO_CLEAN variable, which means
rm -f will not work. We need to use rm -rf.

Change-Id: I4e10d70d5638ce7e7a9215f5bd848422aa90aad3
Closes-Bug: #1335399

bljgaurav/openstack-test,varunarya10/devstack,upadhyay-prashant/devstack,thomasem/devstack,richliu/devstack,pczerkas/devstack,pczerkas/devstack,mmasaki/devstack,TwinkleChawla/devstack,promptworks/devstack,wenhuizhang/devstack,vishnugonela/devstack,flavio-fernandes/devstack,liuquansheng47/devstack,parul24/devstack,kplus/devstack-moonshot,pacinete/devstack,atulpatil301/devstack,vsham20/devstack,affo/devstack,tianweizhang/devstack,virtualopensystems/devstack,CloudServer/devstack,openstack-dev/devstack,herrsechs/devstack,noironetworks/devstack,olivierlemasle/devstack,varunarya10/devstack,pombredanne/devstack,Mahito/devstack,ozamiatin/devstack,atulpatil301/devstack,Mahito/devstack,bq-xiao/devstack,j4/devstack,bigswitch/devstack,bswartz/devstack,nuxeh/devstack,HPCHub/devstack,innodee/openstackk,dirkmueller/devstack,ozamiatin/devstack,dirkmueller/devstack,andrewrothstein/devstack,neerja28/Devstack_GlusterFS,vishnugonela/devstack,eharney/devstack,TwinkleChawla/devstack,j4/devstack,mssumanth/devstack,dtroyer/devstack,vsham20/devstack,group-policy/devstack,bq-xiao/devstack,sequenceiq/devstack,rushiagr/devstack,eshnil2000/devstack,sequenceiq/devstack,BSJAIN92/OpenStack,pombredanne/devstack,prithivm/branch01,kplus/devstack-moonshot,dtroyer/devstack,parul24/devstack,pacinete/devstack,mssumanth/devstack,mmasaki/devstack,CloudServer/devstack,mc2014/devstack,costingalan/devstack,sc68cal/devstack,BSJAIN92/OpenStack,LoHChina/devstack,upadhyay-prashant/devstack,promptworks/devstack,bswartz/devstack,NaokiMiyata/devstack,olivierlemasle/devstack,openstack-dev/devstack,avvocatodemarchis/devstack,flavio-fernandes/devstack,andrewrothstein/devstack,HPCHub/devstack,neerja28/Devstack_GlusterFS,affo/devstack,mc2014/devstack,prithivm/branch01,group-policy/devstack,nawawi/openstack,eharney/devstack,bigswitch/devstack,LoHChina/devstack,rushiagr/devstack,samgoon/devstack,NaokiMiyata/devstack,sc68cal/devstack,herrsechs/devstack,wenhuizhang/devstack,tianweizhang/devstack,liuquansheng47/devstack,samgoon/devstack,noironetworks/devstack,thomasem/devstack,nuxeh/devstack,richliu/devstack,bljgaurav/openstack-test,nawawi/openstack,costingalan/devstack,avvocatodemarchis/devstack