upgrade.sh: add ability to run certain upgraders

Since our upgrade tarball has both upgraders for master node and
openstack upgrader, we should provide a way to run only certain
ones. For instance, it's a very useful workflow when we can run
just openstack upgrader to test openstack patching.

Upgraders to run could be specified throughout special env variable,
which is widespread way to parametrize some process in Unix world:

Usage example:

    $ UPGRADERS='openstack' ./upgrade.sh

The command above will run just openstack upgrader, no matter which
upgraders was hardcoded in upgrade.sh.

Change-Id: I601ad84eba6403dc6a1885b6ea452056a9b667af
Blueprint: upgrade-to-5-1