Fixes #943 (again)

The previous PR #969 for this setting does remove the error message, but it does so by duplicating the default sshd jail a second time with a different name, and does not enable the ddos filters as intended.

Only a single jail is needed, this PR specifies the aggressive mode, which includes the default filters, plus 'extra' and 'ddos' filters, per the instructions in the fail2ban config files:

# To use more aggressive sshd modes set filter parameter "mode" in jail.local:
# normal (default), ddos, extra or aggressive (combines all).
# See "tests/files/logs/sshd" or "filter.d/sshd.conf" for usage example and details.
#mode   = normal

# Parameter "mode": normal (default), ddos, extra or aggressive (combines all)
# Usage example (for jail.local):
#   [sshd]
#   mode = extra