ssh: Use logger; avoid unnecessary string creation

[IngelaAndin]

ERIERL-1010 to be included

Improve ssh application.

@KennethL @IngelaAndin - please review the work item

DoD

1. logger used instead of error_logger
2. if possible string processing is avoided depending on current log level (e.g. info log event content is not prepared if log level is error)
3. plan after OTP-26 release