prove an apt configuration string to use.

Also it's possible to define own key id and/or keyserver to download
the apt key, and/or to provide the apt key within a variable
`clickhouse_repo_key_data`.

Closes: AlexeySetevoi#15

Travis supports only for trusty, so other distros couldn't
be tested.

Now there is a clickhouse_supported_distros list with list
of supported versions in form of string:
'{{ ansible_distribution }}_{{ ansible_distribution_release }}'