Expose mirror update `-max-tries=` parameter in pyaptly conf

[karras]

The new -max-tries= parameter for mirror update cannot be set through the pyaptly config at the moment:

https://www.aptly.info/doc/aptly/mirror/update/

This needs at least aptly version 1.0.0 (https://github.com/smira/aptly/releases/tag/v1.0.0)!