MongoDB Fails to get TGT
I think that you missed one step. You should add an entry into the keytab file for your principal mongodb/mongodb.centos7.vm@CENTOS7.VM

Suppose that the path for your keytab file is /etc/krb5.keytab. Now run $ktutil

ktutil: add_entry -password -p mongodb/mongodb.centos7.vm@CENTOS7.VM -k 1 -e des-cbc-md4
ktuitl: wkt /etc/krb5.keytab 
ktuitl: quit


Now you can run $klist -k /etc/krb5.keytab
and the new entry should appear on the output.

Now, I think you can start mongod service.