Do not try to create a DB if opend in readonly mode

The behavior of IOVDbConn.cxx is to try to create a COOL-database if it can't open it. The conditions-writing jobs rely in this functionality to either amend or create a COOL-sqlite file. But right now, the attempt-to-create happens even if m_readOnly flag is set. This leads to misleading error messages in production jobs. With this MR, the attempt-to-create happens only if the m_readOnly flag is false.

Note that the read-only flag is set IOVDbSvc if there no colon in the connection string, assuming a logical connection string has been used.

While this will not solve the problem described in ATDBOPS-399 (and a lengthy e-mail thread about saturating ATONR-connections) it should avoid very confusing error messages.