octavia: add new pool monitor types

[kayrus]

resolves #2260

[coveralls]

Coverage remained the same at 79.862% when pulling 102e0d9 on kayrus:octavia-new-monitors into 3889f57 on gophercloud:master.

[theopenlab-ci]

Build failed.

• gophercloud-unittest : SUCCESS in 5m 15s
• gophercloud-acceptance-test : FAILURE in 1h 35m 41s
• gophercloud-acceptance-test-ironic : SUCCESS in 1h 04m 21s

[EmilienM]

I confirm that these values are supported by Octavia:
https://github.com/openstack/octavia-lib/blob/master/octavia_lib/common/constants.py#L50-L56

And used by Octavia when creating the listeners.

[EmilienM]

this LGTM and safe to land without the acceptance test.
note that this won't work when the backend is OVN, as it doesn't support Health monitors. However, this would return a proper error if anyone tried to created a health monitor when LB is backed by OVN, so nothing to to in Gophercloud I think.
thanks!