Fix misc nits

[melzhan]

Comments copied from internal code:

third_party/openconfig/bootz/server/entitymanager/entitymanager.go
[349]
encrypt the response

Nit: This, and the following log entry, are incorrect. The message is being signed, not encrypted.

[third_party/openconfig/bootz/server/service/service.go]
[36]
Key

Nit: It's a good practice to explicitly label this as a private key (i.e. PrivateKey). This helps prevents accidentally mixing up what you're dealing with.

See, for example, google3/third_party/go/gc/src/crypto/tls/common.go;l=1408;ws=codereview%2F678366780;rcl=34