Add ssh-keygen key format option

[alaa]

Adding an option to specify the format of the generate SSH keys using ssh-keygen utility. This helps users to better integrate and manage flux installation with existing automation tools which are only able to read certain key format.

For example, Terraform TLS provider is not able to read the default OpenSSH RFC4716 format so being able to change the format on the installation to PEM format solves this issue and makes it trackable by the Terraform state natively.

I kept the default format in the values file the same, so basically this will not change the default format which Flux uses at installation time. But only add the ability to pick different formats when needed.

For more information check the -m option in man ssh-keygen

[hiddeco]

Neat and tidy, could not have done it any better. Thanks @alaa 🌷