thar-be-settings: Add trace debugging to client module

[tjkirch]

This helped solve the question in #406.

Testing done:

Saw the trace messages when I called t-b-s with -v -v. Example:

Oct 14 19:53:11.683 TRACE thar_be_settings::client: GETing from /settings?keys=this,settings.kubernetes.cluster-name,settings.updates.seed,settings.updates.metadata-base-url,settings.kubernetes.max-pods,settings.ntp.time-servers,settings.updates.target-base-url,settings.hostname,settings.kubernetes.cluster-certificate,settings.kubernetes.cluster-dns-ip,settings.kubernetes.pod-infra-container-image,settings.kubernetes.node-ip,settings.kubernetes.api-server