mg-schema-test-database: Change username for back-to-main-db xref

mg-schema-test-database: Change username for back-to-main-db xref

The `username' of the xdbref task in the test db, referring to the
main db, is changed to `PARENT' (from `<username>@<nodename>').

Currently this is purely cosmetic, but it is going to be useful to
distinguish the two cases:
 * This is a test DB and contains references to a parent
 * This is a parent DB (probably the main DB) which contains
   references to child test DB(s).

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v3: Fix `DBS' in commit message to `DB(s)'.
v2: New patch