vty: Use generic 'end' and 'exit' commands

Add bsc_install_default() and replace all install_default()

This patch adds bsc_install_default() which calls install_default()
and add 'exit' and 'end'. All other calls to install_default() are
replaced by calls to bsc_install_default().

Since 'exit' and 'end' are now added automatically to each node, the
explicit registrations of these commands are removed by this patch,
too.

The related tests succeed now without work-arounds (except for the
'config' node itself which is part of libosmocore).
diff --git a/openbsc/src/libcommon/common_vty.c b/openbsc/src/libcommon/common_vty.c
index 7aca4ca..0bf43b0 100644
--- a/openbsc/src/libcommon/common_vty.c
+++ b/openbsc/src/libcommon/common_vty.c
@@ -174,3 +174,13 @@
 		talloc_free(*dst);
 	*dst = talloc_strdup(ctx, newstr);
 }
+
+void bsc_install_default(enum node_type node)
+{
+	install_default (node);
+
+	if (node > CONFIG_NODE) {
+		install_element(node, &ournode_exit_cmd);
+		install_element(node, &ournode_end_cmd);
+	}
+}