Executor can either pull from images.sbgenomics.com or dockerhub but not both

[zymergen-mbarton]

@zymergen-msneddon encountered an issue where, if logged into
images.sbgenomics.com with docker, public docker images could not be pulled
from dockerhub. Logging out allowed these to be then be downloaded but meant
that private SBG images could no longer be downloaded.

[zymergen-msneddon]

To add a little more information, this appears to occur when using the rabix/config/core.properties file to configure docker authentication, setting docker.username, docker. password settings.