Forge tests of status-get command in HA environment
isc-private/qa-dhcp#280
Added _get_status_HA() and included it in existing tests to expand them with 'status-get' testing.
_get_status_HA() checks if server1 and server2 return proper HA dependent values in status-get.
Testing of status-get is added only to some HA tests and also not as a separate test to conserve (long enough) time the HA tests take, especially since status-get tests require testing two communication channels (socket and HTTP) which doubles the required time.
Code tested on Jenkins: https://jenkins.aws.isc.org/view/Kea-manual/job/kea-manual/job/tarball-system-tests/82/
Closes isc-private/qa-dhcp#280