Fix LXD market importer

[dann1]

Replaces #3138

• centos 6 and 7 enable sshd at startup after installing openssh-server
• alpine was missing curl, so it couldn't get the context package
• packages were being left on container /root/ directory

Since we won't add ssh dependency for alpine on context-package, we could install openssh here on alpine case.

[rsmontero]

Consider also my comment to #3138 about custom package installation. This commit does not include ssh installation?

[dann1]

Consider also my comment to #3138 about custom package installation. This commit does not include ssh installation?

Done, we had openssh stated but it was confirmed as a missing dep for context package. I guess until we release a new context version we can state here the missing deps such as openssh and dbus for ubuntu.

@vholer do you think stating openssh here on alpine it is correct, taking into account my unmerged packages PR