Define gerrit_root in copy-plugin-deps

The copy-plugin-deps role relied on an undefined variable "gerrit_root".
This caused plugins such as delete-project, which have external deps,
to fail the gerrit-plugin-build job.  Correct this by defining the variable
as expected.

Change-Id: I68ef0074b69fe3f2e9a7381eb9e9f9984d4eac11