Create Firefox configuration extension on CA-less install

Create:
* kerberosauth.xpi
* krb.js

even when --http_pkcs12 option is used.

https://fedorahosted.org/freeipa/ticket/3747
Enable SASL mapping fallback.

Assign a default priority of 10 to our SASL mappings.

https://fedorahosted.org/freeipa/ticket/3330