pool: Fix pool selection bugs in migration module

There were two bugs in migration module:

- Random pool selection would select pools even when they are full.

- Proportional pool selection would trigger a NPE as it would return
  null when all pools are full.

Target: trunk
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7362/
(cherry picked from commit 55de0de55cc6940f6204dee68e29ec24a13c75d3)

Conflicts:
	modules/dcache/src/main/java/org/dcache/pool/migration/Task.java
Merge pull request #921 from gbehrmann/fix/2.10/rb7362

pool: Fix pool selection bugs in migration module