(PUP-2356) Add additional yumrepo options

This change introduces some new yumrepo options:
 * gpgcakey
 * retries
 * throttle
 * bandwidth
 * mirrorlist_expire

- Added also the possibility to specify dhm units and never as value for
metadata_expire option
Merge branch 'pull-2577'
Merge branch 'master' of https://github.com/puppetlabs/puppet