Prevent infinite Inactivity Timeouts

In previous versions, if the Inactivity Timeout value is set to 0, then Centro would interpret this as an infinite timeout; in other words, the user would never have a timeout requiring to login again.

Centro now prevents the inifite inactivity timeout. Values of 0 result in an error.

Note: For systems upgrading and had previous values set to 0, the value is converted to 600.