s3_repository: Fix panic if YAML is malformed

Fixes the following "capstan search" panic when the YAML file in S3
bucket is malformed:

panic: interface conversion: yaml.Node is yaml.Scalar, not yaml.Map

goroutine 1 [running]:
runtime.panic(0x6c0620, 0xc21008fb40)
	/usr/lib64/golang/src/pkg/runtime/panic.c:266 +0xb6
github.com/cloudius-systems/capstan.yamlToInfo(0xc21008c240, 0x8, 0xc21008c249, 0xd, 0x7f69a3b308d0, ...)
	/home/penberg/go/src/github.com/cloudius-systems/capstan/s3_repository.go:61 +0x67
github.com/cloudius-systems/capstan.RemoteFileInfo(0xc21008c240, 0x21, 0x0)
	/home/penberg/go/src/github.com/cloudius-systems/capstan/s3_repository.go:90 +0x1cb
github.com/cloudius-systems/capstan.ListImagesRemote(0x713480, 0x0)
	/home/penberg/go/src/github.com/cloudius-systems/capstan/s3_repository.go:113 +0x222
main.func·008(0xc21000a6e0)
	/home/penberg/go/src/github.com/cloudius-systems/capstan/capstan/capstan.go:147 +0x9a
github.com/codegangsta/cli.Command.Run(0x722a20, 0x6, 0x0, 0x0, 0x755e90, ...)
	/home/penberg/go/src/github.com/codegangsta/cli/command.go:73 +0x994
github.com/codegangsta/cli.(*App).Run(0xc21007b000, 0xc21000a000, 0x2, 0x2, 0x1, ...)
	/home/penberg/go/src/github.com/codegangsta/cli/app.go:111 +0x855
main.main()
	/home/penberg/go/src/github.com/cloudius-systems/capstan/capstan/capstan.go:151 +0x7c2

goroutine 4 [syscall]:
runtime.goexit()
	/usr/lib64/golang/src/pkg/runtime/proc.c:1394

goroutine 5 [runnable]:
net/http.(*persistConn).readLoop(0xc210060380)
	/usr/lib64/golang/src/pkg/net/http/transport.go:778 +0x68f
created by net/http.(*Transport).dialConn
	/usr/lib64/golang/src/pkg/net/http/transport.go:528 +0x607

goroutine 6 [select]:
net/http.(*persistConn).writeLoop(0xc210060380)
	/usr/lib64/golang/src/pkg/net/http/transport.go:791 +0x271
created by net/http.(*Transport).dialConn
	/usr/lib64/golang/src/pkg/net/http/transport.go:529 +0x61e

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>