Sync DB2 error code handling from oslo

Including DB2 error code -30081 in conn_err_codes, so that the
connection retry code will not be skipped when DB2 connection
error happens.

SHA1 of Oslo commit - bdabd512943ebb84a1333b2b3ee31a33124af76e to
which the db module code is syncd.

Change-Id: Idec7a6d712f5ed08fc0daad0a64df203c624f194
Closes-Bug: #1244092
Merge "Sync DB2 error code handling from oslo"