If cfssl is configured and not installed, the registrar fails with a misleading message
Should you run with the default `ca_implementation = cfssl` and not have golang and cfssl installed, the registrar will fail with the following message:

```
2019-09-25 12:33:18.918 - keylime.cloudverifier_common - INFO - Setting up TLS...

Traceback (most recent call last):

  File "/usr/local/bin/keylime_registrar", line 11, in <module>

    load_entry_point('keylime==1.2', 'console_scripts', 'keylime_registrar')()

  File "/usr/local/lib/python3.7/site-packages/keylime-1.2-py3.7.egg/keylime/registrar.py", line 36, in main

    registrar_common.start(config.getint('general', 'registrar_tls_port'),config.getint('general', 'registrar_port'),config.get('registrar', 'db_filename'))

  File "/usr/local/lib/python3.7/site-packages/keylime-1.2-py3.7.egg/keylime/registrar_common.py", line 434, in start

    generatedir='reg_ca')

  File "/usr/local/lib/python3.7/site-packages/keylime-1.2-py3.7.egg/keylime/cloud_verifier_common.py", line 136, in init_mtls

    raise Exception("It appears that the verifier has not yet created a CA and certificates, please run the verifier first")

Exception: It appears that the verifier has not yet created a CA and certificates, please run the verifier first
```

This is after the verifier has run with the following:

```
2019-09-25 12:32:48.812 - keylime.cloudverifier - INFO - Starting Cloud Verifier (tornado) on port 8881, use <Ctrl-C> to stop
2019-09-25 12:32:48.813 - keylime.cloudverifier_common - INFO - Setting up TLS...
2019-09-25 12:32:48.813 - keylime.cloudverifier_common - INFO - Generating a new CA in /var/lib/keylime/cv_ca and a client certificate for connecting
2019-09-25 12:32:48.814 - keylime.cloudverifier_common - INFO - use keylime_ca -d /var/lib/keylime/cv_ca to manage this CA
2019-09-25 12:32:48.814 - keylime.cloudverifier_common - WARNING - CAUTION: using default password for CA, please set private_key_pw to a strong password
```

We basically need to check somehow, that if cfssl is configured as the ca implementation, is it even present to be used?
diff --git a/keylime/ca_impl_cfssl.py b/keylime/ca_impl_cfssl.py
index b512665..117c171 100644
--- a/keylime/ca_impl_cfssl.py
+++ b/keylime/ca_impl_cfssl.py
@@ -25,6 +25,8 @@ import subprocess
 import socket
 import time
 import requests
+import shutil
+import sys
 
 try:
     import simplejson as json
@@ -68,6 +70,9 @@ def post_cfssl(params,data):
     return response.json()
 
 def start_cfssl(cmdline=""):
+    if shutil.which("cfssl") is None:
+        logger.error("cfssl binary not found in the path.  Please install cfssl or change the setting \\"ca_implementation\\" in keylime.conf")
+        sys.exit(1)
     global cfsslproc
     cmd = "cfssl serve -loglevel=1 %s "%cmdline
     env = os.environ.copy()
['keylime/ca_impl_cfssl.py']