To Reproduce

When creating a HTTP/HTTPS Health-Monitor, Octavia doesn't allow to configure the content of the host field header, and doesn't sent any value as well. 

Hence, Health-Monitors based on the HTTP/1.1 and HTTP/2.0 do not work properly because the Host header field MUST be sent in all HTTP/1.1 and HTTP/2.0 request messages (as described in RFC2616 for HTTP/1.1 : https://www.rfc-editor.org/rfc/rfc2616#section-14.23). A 400 Bad Request status code may be sent to any HTTP/1.1 and HTTP/2.0 request message that lacks Host header field.

In summary, we need Octavia to be aligned with the RFCs and allow to configure the host field in the header for HTTP/HTTPS Health-Monitor.

Expected behavior

• add a field in Octivia to set Host in query of Health-Monitor or more configuration Haproxy backend can manage

Device Info (please complete the following information):

• OS Version: RHOSO 18.0.4

Bug impact

• Impact the onboarding of customers that uses HTTP/1.1 and HTTP/2.0 with HTTP based Health-Monitor