Add syslog support for vhosts

This commit adds syslog support as logging parameter for error and
access log files.

Signed-off-by: Marc Koderer <m.koderer@telekom.de>
Add tests and documentation for syslog parameter.

Signed-off-by: Marc Koderer <m.koderer@telekom.de>
Merge pull request #324 from TelekomCloud/feature/syslog

Add syslog support for vhosts