Expose log retain time

[mzmssg]

1. enabled the log retain time as configurable settings for Admin.
2. extend job log retention time from 7 days to 30 days..
3. add this PR: decouple data-node image from cluster object module #1870, otherwise paictl will try to render the host_ip in advance of generate_script

[coveralls]

Coverage increased (+0.01%) to 52.658% when pulling 8a725b1 on zimiao/expose_log_retain into 6b8f2d4 on master.