common: add str_join helper

Signed-off-by: Sage Weil <sage@inktank.com>
config: fix stringification of config values

The std::copy construct leaves a trailing separator character, which breaks
parsing for booleans (among other things) and probably mangles everything
else too.

Backport: dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
mon: use str_join instead of std::copy

The std::copy method leaves a trailing separator.

Signed-off-by: Sage Weil <sage@inktank.com>