LBaaS: check for associations before deleting health monitor

Need to prohibit health monitor deletion if it has associations with
pools. Given that pools may belong to different lbaas drivers the process
of monitor deletion becomes complex and unreliable since association
deletion may fail on any single driver.

DocImpact

Closes-Bug: #1243129
Change-Id: I27c20e7a5be8433f90569534ecf838e33027cb00
Merge "LBaaS: check for associations before deleting health monitor"