build-and-provide-package: support setting custom reprepro options via REPREPRO_OPTS

This is useful e.g. when you want to keep older versions of
package files available when needed. This is possible e.g. via:

  export REPREPRO_OPTS='-v --waitforlock 1000 --keepunreferencedfiles'

This avoids hitting the race condition when you execute "apt-get
update ; apt-get install $package" but between the update and the
package installation processes there have been changes on the
reprepro mirror side and the requested package no longer can be
downloaded because the file has been gone already.

NOTE: to get rid of the unreferenced files again on demand
execute "reprepro -b $REPOSITORY deleteunreferenced".

Development sponsored by Sipwise GmbH