Let paths to roles be relative to playbook if started with '.'

Currently, if you want to point to a role directory, you need to use
absolute paths (which isn't ideal); if you use a relative path, then
that is always relative to the roles/ subdirectory.

If you want to include a role that is in a subdirectory of the
playbook, e.g. 'foo/bar', then you need to use '..':

  ../foo/bar   →   basedir/roles/../foo/bar   →   basedir/foo/bar

This commit changes the origin for the resolution of the relative path
to the directory of the playbook, if the rolename starts with a '.':

  ./foo/bar   →   basedir/foo/bar

Current users just keep on saying 'foo' to get at 'roles/foo'

But if you say e.g. './foo', then this means './foo', not
'/roles/foo'.

Signed-off-by: martin f. krafft <madduck@madduck.net>