common: multiple variable occurence expansion test

http://tracker.ceph.com/issues/7103 refs #7103

Signed-off-by: Loic Dachary <loic@dachary.org>
common: cosmetic inversion of tests arguments

Signed-off-by: Loic Dachary <loic@dachary.org>
common: recursive implementation of config::expand_meta

Using a recursive implementation of variable expansions make it easier
to protect against loops and provide human readable messages when they
happen.

It also enables one variable to be substituted multiple times in the
same configuration option instead of just once because it is confused
with a variable expansion loop.

http://tracker.ceph.com/issues/7103 fixes: #7103

Signed-off-by: Loic Dachary <loic@dachary.org>
common: unit tests for config::expand_meta

Part of the config.cc tests are in test/confutils.cc but they do not
cover meta variable expansion. Create unittest_config for config.{h,cc}
specific tests.

The test_md_config_t is made a friend of md_config_t to allow testing
private and protected methods.

test/cli/ceph-conf/show-config-value.t is used to check that the human
readable message message shows as expected when there is an expansion
loop.

Signed-off-by: Loic Dachary <loic@dachary.org>