Do not attempt creating ssh config files if ssh_config.d doesn't exist

[hellt]

reported by a user:

INFO[0025] Adding containerlab host entries to /etc/hosts file 
INFO[0025] Adding ssh config for containerlab nodes     
ERRO[0025] failed to create ssh config file: open /etc/ssh/ssh_config.d/clab-srl01.conf: no such file or directory 

this means the path did not exist and this PR ensures the path exists.
It might be that the OS user runs or OpenSSH there won't honor the newly created path either, but it seems still safe to create the path

[steiler]

Hmm my feeling is more like, if that path does not exist, we do not generate the config rather then generating something that is not used anyways.
Another option could be to add an Include to the /etc/ssh/ssh_config and create a /etc/clab/ssh_config.d directory anyways, like even when the /etc/ssh/ssh_config.d exists.

[hellt]

I will ask the user to create this file and see if his ssh client picks it up

[hellt]

@steiler re-applied the patch to follow the suggested logic, as old ssh clients do not respect the created path.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@1784608). Click here to learn what that means.
The diff coverage is 50.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1708   +/-   ##
=======================================
  Coverage        ?   51.60%           
=======================================
  Files           ?      141           
  Lines           ?    13751           
  Branches        ?        0           
=======================================
  Hits            ?     7096           
  Misses          ?     5867           
  Partials        ?      788           

Files	Coverage Δ
cmd/deploy.go	64.62% <100.00%> (ø)
clab/sshconfig.go	63.63% <42.85%> (ø)