P12 Key: allow providing the private key directly

Different versions of openssl support different P12 key formats. The
key from google wasn't supported by the version of openssl we were
using so it was easier to allow supplying the private key directly.

In particular, the key worked in our staging environment, but in production
we received **this error**: 
```
Unable to parse the p12 file. Is this a .p12 file?  Is the password correct?  OpenSSL error: 
```
*notice the openssl error string is blank, which was not helpful*