An EXTRA_CIME_CONFIGS env var

Problem: a group of users are running a CIME model on a machine that is NOT in the code's config_machines.xml file (because they aren't main developers or its a secure machine only a few people can get on.

They could use the ~/.cime feature to define their machine in their user dir but they would have to use some other mechanism to make sure they all have the same version.

An EXTRA_CIME_CONFIGS env var could point to a shared directory where all users could find one official copy of the config for that machine.

Start by tracing how CIME loads config_machines.xml and how the ~/.cime user configuration is discovered. Identify the existing machine-configuration entry point and add coverage for an EXTRA_CIME_CONFIGS directory; done means users can load a shared machine configuration without editing the main repository file.

Written by the indexing model from the issue text.