“Steel Magnolias” by Robert Harling exists in that narrow corridor between sentiment and sentimentality. When not carefully directed and acted, it offers opportunities for maudlin comments on the unfairness of life. But in a solid production, like the Keegan Theatre’s current offering, honest feeling prevails over bathos and banality. Although there is a central character in “Steel Magnolias,” a vibrant young woman named Shelby (Laura Herren), introduced on her wedding day, the play is very much about the five characters who surround her.