set default v3 extension profile

[flihp]

Each time OKS runs it uses the Purpose enum from the keyspec to select the v3_exteion section to use when signing certs. This is effectively a default since the keyspec is saved to the CA directory and shouldn't change. Might be cleaner to set the x509_extensions config key in the openssl.cnf instead. This will also make use of the CA by non-oks tools (ones that are ignorant of KeySpecs) less difficult / awkward.