The default values of fields in both ComposerConfig.Koji and
ComposerConfig.Worker are well-suited for how they're used.
The nil-checks in main.go only checked that the sections exist. This is
quite a weak check for validity, because the sections could be empty. If
anything is required for composer to function, we could add proper
validation in the future.
Do the same for the CA fields, which contain file names. Go has lots of
precedent for using empty strings to denote "no value" in the standard
library. Use it for CA files, too, instead of pointers.
The configuration file is API. Let's give it a bit more prominence to
help people treat it as such, and a chance to test it. A basic test is
included in this commit.
Also, this cuts down on the noise in main.go a bit.
