Checks if the provided atSign's atServer has been properly activated with a public RSA key.
atClient must be authenticated
atSign is the atSign to check
Returns true, if the atSign's cloud secondary server has an existing public:publickey@ in their server,
Returns false, if the atSign's cloud secondary exists, but does not have an existing public:publickey@
Throws AtClientException if the cloud secondary is invalid or not reachable
Returns deviceName with uppercase latin replaced by lowercase, and
whitespace replaced with underscores. Note that multiple consecutive
whitespace characters will be replaced by a single underscore.