common: evaluate --show-config* after CEPH_ARGS

The content of CEPH_ARGS is appended to the list of arguments. When
--show-config or --show-config-value is also set, it should be evaluated
after all arguments are parsed to accurately reflect the value that
would be visible to the program.

It failed to do so because the action for --show-config* was carried out
immediately. It is postponed until all options are parsed instead.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1019 from dachary/wip-show-config-value

common: evaluate --show-config* after CEPH_ARGS

Reviewed-by: Sage Weil <sage@inktank.com>