Rename auth binaries to match conventions in other Istio repos

[ZackButcher]

Auth's binaries all use underscores:

• generate_cert
• generate_csr
• istio_ca
• node_agent

Across the rest of Istio we don't do this:

• pilot
• agent
• mixs
• mixc
• mixcol
• istioctl
• sidecar-initializer

In 0.3 Auth should rename binaries to match the conventions of the rest of the project. Plus underscores in a CLI are a real pain to type compared to no underscore or a dash.

IMO gencert, gencsr, istiocas (Istio CA Server, or just istioca), and agent (or auth-agent) seem more in line with the rest of Istio.

[wattli]

sgtm