ssh-admin: quieten noisy logging

We deliver dCache without an authorized_keys2 file, yet
attempts to log in when a user presents an SSH key results in
logging like:

03 Jul 2013 14:11:40 (ssh2Admin) [] The authorized_keys2 file was not found. Please check if it exists in: /home/paul/git/dCache/packages/system-test/target/dcache/etc/admin/authorized_keys2
03 Jul 2013 14:11:40 (ssh2Admin) [] Could not find key: RSA Public Key
            modulus: b724ef9e0182b02db76ea20a0f674d83 [many hex digits follow ...]
    public exponent: 23
in /home/paul/git/dCache/packages/system-test/target/dcache/etc/admin/authorized_keys2.
03 Jul 2013 14:11:40 (ssh2Admin) [] The authorized_keys2 file was not found. Please check if it exists in: /home/paul/git/dCache/packages/system-test/target/dcache/etc/admin/authorized_keys2
03 Jul 2013 14:11:40 (ssh2Admin) [] Could not find key: DSA Public Key
            y: 51e74d6deb9b0b87456522d56f4fb952c21ec0 [many hex digits follow ...]
in /home/paul/git/dCache/packages/system-test/target/dcache/etc/admin/authorized_keys2.

This is wrong as the authorized_keys2 file is not required and
the login attempt was actually successful, using username+password
authentication.

This patch reduces the above messages to a single message per failure to
read the authorized_keys2 file (rather than two) and logs this message at
debug level, rather than error.

Some log messages were also rephrased to avoid unnecessary verbosity.

Target: master
Requires-book: no
Requires-notes: no
Patch: http://rb.dcache.org/r/5696/
Acked-by: Gerd Behrmann