Merge pull request #17539 from tchaikov/wip-21204

mon,option: set default value for mon_dns_srv_name

Reviewed-by: Sage Weil <sage@redhat.com>

diff --cc src/ceph_mon.cc
Simple merge

diff --cc src/common/legacy_config_opts.h
Simple merge

diff --cc src/common/options.cc
Simple merge