Fix 'Associate' and 'Disassociate Monitor' actions

Since context['pool_monitors'] is not a list of monitors' ids anymore,
lists of possible monitors choices were broken. Now 'Associate Monitor'
and 'Disassociate Monitor' forms show proper health monitors lists for
associating/disaccociating.

Closes-Bug: #1351572
Change-Id: If496652ac082ad9fa2beba26bc7f6849ebf4cc14
Merge "Fix 'Associate' and 'Disassociate Monitor' actions"