Add GEO_FAILOVER variable to set gitlab address for failover tests

When testing failover on staging where a Geo secondary site is promoted to a Geo primary (i.e. read/write) site, we want to run tests against the secondary to make sure all read/write features work correctly. We want to do this before updating the Geo primary DNS record to point to the newly promoted secondary.

The current staging scenarios hard-code GITLAB_ADDRESS to staging.gitlab.com. Now, by setting new env var 'GEO_FAILOVER' to true, we can specify the Geo secondary site address (geo.staging.gitlab.com) as GITLAB_ADDRESS.

I also added shared examples to env_spec.rb as suggested by a comment in the spec file.