Rename the `location` variable to `subnet`

This makes it clearer what the intent of this variable is, after the discussion in infra/puppet/puppet-environment!34

Depends on !66 (closed)

Test Plan

Need to add a subnet fact to the data of hosts in octocatalog-diff.

Something along the lines of:

sed -i 's/^  location: \(.*\)$/&\n  subnet: \1/' octocatalog-diff/facts/*.yaml

in the puppet-environment repo

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Migrated from D4164 (view on Phabricator)