Bug 1471044 - Allow some model classes to have dynamic column names with class metho...

Bug 1471044 - Allow some model classes to have dynamic column names  with class method DYNAMIC_COLUMNS

When working on bug 1253535 I kept encountering the fragility of our model system.

This fragility involves two things:

1) Extensions adding new columns to model objects
2) The use of those model objects in checksetup.

In particular, the SecureMail extension causes this problem because (at checksetup-time) we need to lookup certain Group objects.

Thus I propose to

1) allow model classes to take their list of columns from the informations schema (exposed via `bz_table_columns_real`)
2) Have Bugzilla::Group do this when being used in a commandline (e.g. checksetup) context.