.gitignore: ignore *.cer, *.crt, *.key, *.pem and *.img

• .cer, .crt, .key and .pem are typical file name extensions for TLS certificates and keys. They are used when codesigning with openssl and should never be commited in the repo.
• .img is a generic image file. Such files could be used when repacking an ISO to attach cloud-init CIDATA.