mgr/cephadm: add extra_container_args and custom_configs to CustomContainer

mgr/cephadm: add extra_container_args and custom_configs to CustomContainer

CustomContainer was skipped previously for the extra_container_args
and custom_configs feature as these could already be done
using other fields within the custom container service spec
(the "args" and "files" fields respectively). It seems
desirable for us to allow setting these things for custom
containers the same as for other services for uniformity sake
and this allows us to use custom containers to test
these features.

Signed-off-by: Adam King <adking@redhat.com>
(cherry picked from commit b66892a668d487f4b7ede147eb8855d166c3d1f9)