Honor env vars when using builder

Previously, we would honor HELIOS_DOMAIN and HELIOS_ENDPOINTS
only if the caller used TemporaryJobs.create(). We now honor
those variables even if the caller uses the builder directly.
They can then override those values with hardcoded ones using
builder methods.

-RN
TemporaryJobs builder object now honors HELIOS_DOMAIN and
HELIOS_ENPOINTS.Previously they were honored only if the
caller used TemporaryJobs.create().
Merge pull request #248 from spotify/rculbertson/temp-jobs-create

Honor env vars when using builder