add cloudconfig command

[melato]

lxops has the functionality to apply cloud-config files to instances, but it does so through an lxops config file.

I propose adding a command to apply cloud-config files directly from the command line:

lxops apply -i <instance> -ostype alpine <cloud-config-file.cfg> ...

lxops apply -i <instance> -ostype alpine - <<END
#cloud-config
runcmd:
- hostname
END

There are currently separate projects cloudconfiglxd, cloudconfigincus that do this. Incorporating this functionality in lxops will have the advantages:

• avoid having to compile cloudconfiglxd and/or cloudconfigincus separately from lxops_lxd, lxops_incus.
• use a unified and consistent command line interface, in lxops instead of duplicating it in cloudconfiglxd, cloudconfigincus.

[melato]

Added instance cloudconfig command. Also added ostypes command to list available os types.