Add unique constraint to ConsolePool.

Added unique constraint to ConsolePool model
`uniq_console_pools0host0console_type0compute_host0deleted` ('host',
'console_type', 'compute_host', 'deleted').
Added new exception `ConsolePoolExists`. Function `console_pool_create`
updated. Tests updated respectively.

blueprint db-enforce-unique-keys
Fixes bug 1185450

Change-Id: I095a52f2d21ec48b079e61034d80bcb9e7fcab79
Merge "Add unique constraint to ConsolePool."