It actually slows down things dramatically when used in combination
with Ansible.

Some systems (e.g. CentOS on Azure) have requiretty in sudoers which makes
pipelining fail.

Add tags to allow more granular tasks filtering.
Add generator script for MD formatted tags found.
Add docs for tags how-to.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>

Moved the variable setting to the apt-get install part where it matters as requested in the review.