Added hostIdentifier to info output

Small change that adds the the hostIdentifier in the output to the info log when running queries.

Also, wraps the `DBHandle::getInstance()` call in a try block, an cause the `getHostIdentifier()` function to throw an error if it cant access the DB.
`getHostIdentifier` accepts a constant and a singleton as it's arguments. that needs love, I'll fix that.

how do you feel about:

```cpp
std::string hostIdentifier;
auto s = getHostIdentifier(hostIdentifier);
if (!s.ok()) {
  LOG(ERROR) << "OMG THE END IS HERE";
}
```

Then we could change the signature of `getHostIdentifier` to:

```cpp
osquery::Status getHostIdentifier(std::string& ident);
```