core: Extract hardcoded strings to constants

This patch pulls hardcoded strings in PoolConditionFieldAutoCompleter to
constants. They are used in follow up frontend patch (sorting columns in
Pools main tab in webadmin application).

Change-Id: I5ab8dc498dc7d8b0e797a26676c3bf92139a9e65
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1104790