Update 'reprepro' to work with many repositories

This patch fixes issues in 'reprepro' role with regards to multiple
repository support and adds new 'wheezy-backports' local repository
which will be used for automatically generated, backported .deb
packages. Existing 'wheezy' repository should be used for manually added
.deb packages.
Add 'backporter' role

'backporter' role can ensure that a package with specified version is
available in APT repository. If it's not available, backporter will try
to build and install available package from newer release.
Drop support for .deb build in 'lxc' role

LXC 1.0 package build on Debian Wheezy will be handled by
'ginas.backporter' role (used as a dependency).
Update CHANGELOG.md
Change the way CA symlinks are handled in 'pki'

I'm not sure what happened here since this set of tasks worked for so
long, but OK, I can update. 'pki' role needs to be revisited at some
point anyway...
Temporarily use Ansible v1.6.6 for Travis-CI tests

This change is due to the security fix in Ansible v1.6.7 and problems
with old playbook format used in ginas. More information:
https://github.com/ansible/ansible/issues/8233

This is a temporary change to allow currently working code in a pull
request to be merged cleanly into main repository.
Merge pull request #106 from drybjed/backporter

Add support for automatically backported packages