cubicweb #5618976 Allow to not auto start an instance when running 'cubicweb-ctl start' [in-progress]

do this by adding a 'start.conf' file in the instance config directory which should contain on word among 'auto', 'manual' or 'disabled'.

'auto': the instance will be started by a "cubicweb-ctl start" (thus by the init script), 'manual': the instance will not be started by a "cubicweb-ctl start" but will be started by "cubicweb-ctl start <appid>" 'disabled: the instance will never be started

done in<not specified>
load left0.250
closed by<not specified>
patch[cwctl] add support for a "start.conf" file (closes #5618976) [reviewed]