If there is a config variable ADCS_TEMPLATE_<upper(authority.name)> take the value as Cert template else default to ADCS_TEMPLATE to be compatible with former versions
```
Traceback (most recent call last):
File "/home/travis/build/Netflix/lemur/lemur/plugins/lemur_acme/tests/test_acme.py", line 159, in test_request_certificate
self.acme.request_certificate(mock_acme, [], mock_order)
File "/home/travis/build/Netflix/lemur/lemur/plugins/lemur_acme/plugin.py", line 211, in request_certificate
current_app.config.get("IDENTRUST_CROSS_SIGNED_LE_ICA_EXPIRATION_DATE", "17/03/21"), '%d/%m/%y'):
TypeError: strptime() argument 1 must be str, not MagicMock
```
https://letsencrypt.org/certificates/
Let's Encrypt is transitioning to use the intermediate CA issued by their own root (ISRG X1) starting from September 29th 2020. This is in preparation of concluding the initial bootstrapping of their CA, by having it cross-signed by an older CA.
https://letsencrypt.org/2019/04/15/transitioning-to-isrg-root.html
This PR allows Lemur to pin to the cross-signed ICA (same public/private key pair as the ICA signed by ISRG X1). This will prolong support for incompatible systems.
If we supply a CSR that contains an empty field in the Subject, Lemur will crash with an error 500 as the ValueError exception is not captured. This change captures the exception and raises a ValidationError which in this case is a 400 sent back to client. Example to reproduce:
Subject: C=ZZ, ST=Something, L=, O=My_Org, OU=My_Dept, CN=www.booking.com
The empty L= causes a ValueError which needs to be captured.
