casts hosts value to []string

Instead of casting the hosts value to []interface{} in FilterByHosts (which
doesnt work because user cannot cast interface{} to []interface{}), this patch
casts host value to []string directly.

Without the patch, running `go run gencert.go` in tlsdemo will panic
because filtered hosts are empty after casting.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)