http 500 error if CSR presented with no CN

[Dave Walker]

It is possible to create a CSR with the omission of a CN. This currently throws a 500 as it is an unhanded error. We can't work with a CSR like this, so we should handle this and throw a 400 Bad Request instead.

2015-06-15 16:46:45,827 ERROR [anchor.certificate_ops][32986/MainThread] Error running validator <server_group> - list index out of range
Traceback (most recent call last):
  File "/home/ubuntu/anchor/anchor/certificate_ops.py", line 113, in validate_csr
    valid = _run_validator(vname, validator, args)
  File "/home/ubuntu/anchor/anchor/certificate_ops.py", line 79, in _run_validator
    validator(**new_kwargs)
  File "/home/ubuntu/anchor/anchor/validators.py", line 177, in server_group
    cn = csr_get_cn(csr)
  File "/home/ubuntu/anchor/anchor/validators.py", line 30, in csr_get_cn
    return data[0].get_value()
IndexError: list index out of range
2015-06-15 16:46:45,828 INFO [wsgi][32986/MainThread] 127.0.0.1 - - [15/Jun/2015:16:46:44 +0000] "POST /sign HTTP/1.1" 500 303 "-" "curl/7.35.0"

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/191899