core: Extract hardcoded strings to constants

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

Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Change-Id: Ic5b840c5b44829d8e6e68d4839977e01258076ff
Bug-Url: https://bugzilla.redhat.com/1104784