task: mon_clock_skew_check: missing 'str'.format() key crashed the test

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
task: mon_clock_skew_check: by default, use max skew from global config

If not defined, defaults to 0.05;  if 'max-skew' however is defined, it
must override whatever is on the config.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>