wrong error message in DBDuplicateEntry exception

[Viktor Serhieiev]

We found that current constraint name convention allows us create
constraints with duplicate names. It happens if we will add constraints
in different tables with same column names (for example, `name`,
`deleted`). In this case we can not create new constraint due to mysql
limitation (it requires unique constraint name for database)
To solve this issue unique constraint name convention was changed
from "uniq_c1_x_c2_x_c3" to "uniq_t$c1$c2$c3" where `t` it is table name
and columns `c1`, `c2`, `c3` are in UniqueConstraint.

Also we need change `_raise_if_duplicate_entry_error()` function respectively to get in error message only column names instead of table name and column names.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/29888