Update dependencies for LXD market images

[dann1]

It would be nice to have these deps installed by context package instead of lxd_downloader.sh

[vholer]

• epel-release can't be a dependency 1) this package exists just in CentOS and not RHEL 2) it must be installed or configured separately before, so the package dependencies from EPEL can be fullfilled on context package install.

• For Alpine, the are 2 main options for the SSH server (OpenSSH and Dropbear), and it's not good to force all users to use OpenSSH only because it's missing a very basic LXD containers. I'm not sure if we can handle alternatives in the package.

• Why is SSH client required? (It's good to distinguish between the required and nice-to-have tools, we should address with contextualization only the minimum required tools).

I would propose to manage the list of things we need (should) add as part of #145 and change it at once before next context release, instead of doing several small ad-hoc changes like this. I'm not going to merge it.