mgcp-client: Introduce API mgcp_client_pool_config_write

It is not a good idea to use a personal write_config function for the
MGW node, since it's not meant to be a 1st level (top) node.
During write-config the node function will be called at an
implementation specific time (based on the order defined by the app in
the node type enum), so there's no real way to control that the output
of the MGW node will end up under the parent configured in
mgcp_client_pool_vty_init(). As mentioned the order can be tweaked based
on how the node enums are configured, installed, etc. but that's really
a nightmare, so let's better not rely on that.

Therefore, this patch introduces a new API which the users (apps such as
OsmoBSc and OsmoMSC) can use to write the config at the required time
(when writing its own parent node).
A hack is introduced internally to detect older versions of the users
which didn't call this function to still print stuff (up to to the app
if the order is correct).

Related: SYS#5987
Change-Id: I7a620cf47886d8ecab30ce369cf123d98ab842c5
---
M TODO-RELEASE
M include/osmocom/mgcp_client/mgcp_client_pool.h
M src/libosmo-mgcp-client/mgcp_client_vty.c
3 files changed, 47 insertions(+), 13 deletions(-)