CmkKpiStatCard: compare the delta against a configurable basis

Replaces deltaRatio with comparisonBasis (average/last/minimum/maximum/
median), computed client-side from the real samples in the displayed
series. Delta stays hidden below two real samples or a zero basis.
show_delta becomes an explicit showDelta prop.

comparisonBasis only holds up when the headline value is itself the
series' own last sample (e.g. CPU utilization). Network-flow's KPI
metrics are window-wide aggregates, so CmkKpiStatCard also gains a
caller-supplied delta prop that takes priority over comparisonBasis;
DashboardContentNetworkFlowKpiStatCard.vue uses it for all nine
metrics, computing percent/direction from the API's previous_value.

CMK-38082

Change-Id: I446dbc1b4f7235a800ca143bcb7f612f8b4af38b