Allow user to connect by specifying either GUID or IP address.

The code allows for using either a GUID or IP address to connect to a camera (lines 103 -- 118 of `src/vimba_ros.cpp`). However, because of the defaults given by the configuration file, IP address always has to be set after which the GUID if supplied (which is done by the configuration file) will be checked. These configuration variables do not really allow for sensible default values.

This pull request sets the default values for `guid` and `ip_address` to the empty string. By doing so, the software will look for user-specified values for these two values, and allows it when only one of the two are set. If both are set, both still have to be correct.