cluster plugin: add obfuscation of luci secrets

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
cluster plugin: restrict data collected for luci

Notably, avoid server cert being added.  On the other hand, allow
collection of rotated log files for luci.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
Merge pull request #168 from jnpkrn/patch-luci

cluster plugin: restrictions wrt. luci