Added syntax help for autotest.sh

autotest.sh now checks for the validity of the db config name argument.
If the db config name is not known, it show syntax info.

This should save some time to people to understand how to use this
script. Before this fix, an invalid argument would return a big HTML
page in the output.
Merge pull request #5537 from owncloud/autotest-syntaxhelp

Added syntax help for autotest.sh