Ticket 47620 - Config value validation improvement
    
    Bug Description:  When setting the replication protocol timeout, it is possible
                      to set a negative number(it should be rejected), and when
                      setting the timeout for an agreement using letters, we get an
                      invalid syntax error, but it should really be an error 53 to
                      be consistent with how the invalid timeout error that is given
                      when updating the replica entry.
    
    Fix Description:  In the agmt modify code, we did not have the actual modify value
                      during the validation.  This allowed the value to be added, which
                      was later caught for the invalid syntax.  Then improved the overall
                      logic to the validation to also catch the negative numbers.
    
    https://fedorahosted.org/389/ticket/47620
    
    Reviewed by: rmeggins(Thanks!)
    (cherry picked from commit 8a4bbc7c74a6847d75e4d6e9e0b16859a5da8ec0)
    (cherry picked from commit 1bbb27b522dd8eb36f09f47c144fd65511c132b5)