lib, tools: fix reloading of key sub-context in key chains

lib, tools: fix reloading of key sub-context in key chains

When you add a key chain in the RIP configuration file and reload the
configurations via the frr-reload.py script, the script will fail and
the key chain will not appear in the running configuration. The reason
is that frr-reload.py doesn't recognize key as a sub-context.

Before this change, keys were generated this way:

    key chain test
     key 2
      key-string 123
     key 3
      key-string 456

With this change, keys will be generated this way:

    key chain test
     key 2
      key-string 123
     exit
     key 3
      key-string 456
     exit

This will allow frr-reload.py to see the key sub-context and correctly
reload them.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>