Add various optional parameters

[mmetz-isa]

Hi,

this PR makes the following optional keepalive parameters truly optional:

• virtual_server.real_server.TCP_CHECK.connect_timeout (Ref: https://github.com/acassen/keepalived/blob/v2.2.4/doc/man/man5/keepalived.conf.5.in#L2291-L2293)
• virtual.server.ip_family (Ref: https://github.com/acassen/keepalived/blob/v2.2.4/doc/man/man5/keepalived.conf.5.in#L2188-L2190)

it also adds the following optional parameter:

• virtual_server.persistence_timeout (Ref: https://github.com/acassen/keepalived/blob/v2.2.4/doc/man/man5/keepalived.conf.5.in#L2208-L2210)

This PR is based on top the 5.0.1 release tag.

As there is no CONTRIBUTE document or section in the README, please feel free to point out any adoptions required for merging this PR.

[evrardjp]

I am fine with those changes.

I think it would be nice to increase coverage of the minimum use case for vserver, as I don't use that, and it has been community contributed.