add macos DMG upgrade

[dothebart]

the dmg file is mounted. This needs to be done with the upgraded version too.

[dothebart]

cleanup needs to unmount all available dmg's .
to achieve this, similar to
#74
un_install_package should get a new boolean force parameter, that defaults to false.
cleanup.py then can set this force to true.
This if condition should choose the first branch if force is specified:
https://github.com/arangodb/release-test-automation/blob/master/release_tester/arangodb/installers/mac.py#L214
and thus unmount all dmg's.

[dothebart]

implementation done.