[fule-library] Use HTTP health-checks in Haproxy

[Aleksandr Didenko]

Currently we don't use http checks (L7) for the most of backends in Haproxy, so it uses TCP connection checks (L4) by default. Switching to HTTP checks should improve backend problem detection and thus improve services availability.

Currently only 'horizon' and 'mysql' backends use L7 checks.

For the following backends:

cinder-api
glance-api
heat-api
heat-api-cfn
heat-api-cloudwatch
keystone-1
keystone-2
neutron
nova-api-2
nova-metadata-api

it could be enabled with the following options right away (tested on live Juno env):

option => ['httpchk', 'httplog','httpclose'],

In addition I suggest to use these options to lower HTTP check rate (default interval is 2s):

balancermember_options => inter 10s fastinter 2s downinter 3s rise 3 fall 3