feat: adds useful info to logs

[EverlastingBugstopper]

• environment variables print with values when accessed
  do not print at all if they do not exist

• each response from rover-client is logged

• some misc. refactors/cleanup associated with the above changes

Before

Feb 12 13:21:00.503 DEBUG rover::utils::env: reading APOLLO_CONFIG_HOME
Feb 12 13:21:00.503 DEBUG rover::utils::env: reading APOLLO_KEY
Feb 12 13:21:00.504 DEBUG rover::utils::env: reading APOLLO_TELEMETRY_DISABLED
Feb 12 13:21:00.504 DEBUG rover::utils::env: reading APOLLO_TELEMETRY_URL
Feb 12 13:21:00.504 DEBUG rover::utils::env: reading APOLLO_REGISTRY_URL
Feb 12 13:21:00.504 DEBUG rover::utils::env: reading APOLLO_CONFIG_HOME
Feb 12 13:21:00.504 DEBUG rover::utils::env: reading APOLLO_KEY
Feb 12 13:21:00.510 DEBUG rover::utils::env: reading APOLLO_VCS_BRANCH
Feb 12 13:21:00.510 DEBUG rover::utils::env: reading APOLLO_VCS_COMMIT
Feb 12 13:21:00.510 DEBUG rover::utils::env: reading APOLLO_VCS_COMMITTER
Feb 12 13:21:00.511 DEBUG rover::utils::env: reading APOLLO_VCS_REMOTE_URL
Feb 12 13:21:00.515 DEBUG houston::profile: APOLLO_KEY=Some("user******************oUPA")

After

Feb 12 13:20:19.555 DEBUG rover::utils::env: read environment variable $APOLLO_KEY = user******************oUPA
Feb 12 13:20:19.577 DEBUG houston::profile: using API key user******************oUPA   
Feb 12 13:20:19.873 DEBUG rover_client::blocking::client: {"data":{"frontendUrlRoot":"https://studio.apollographql.com","service":{"implementingServices":{"__typename":"FederatedImplementingServices","services":[{"name":"people","url":"","updatedAt":"2020-12-18T18:25:44.844Z"},{"name":"films","url":"","updatedAt":"2020-12-18T18:26:10.961Z"}]}}}}

[JakeDawkins]

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