Chris Willman Senior Music Writer and Chief Music Critic Kelsea Ballerini walks into the penthouse suite of a West Hollywood hotel bearing a big smile, a full bottle of wine and two glasses, and this is immediately a good sign for an interview. She has a lot to toast, having just released a well-reviewed album, “Subject to Change,” and being about to play a venue she’s dreamed of headlining, L.A.’s Greek Theatre. Ballerini is coming off a previous album cycle in which she had a No. 2 country hit with “Hole in the Bottle” and a No. 1 with “Half of My Hometown,” which picked up a couple of CMA Awards. But the wine may not be for purely celebratory purposes. “Interviews make me nervous,” she admits, settling in and pouring.