samba-tool visualize: simplify --color-scheme calculations

samba-tool visualize: simplify --color-scheme calculations

If you ask for a --color-scheme, you are implicitly asking for --color.
That was documented in --help, but not followed here.

Now --color=no --color-scheme=ansi will use colour for the graph, but not
for other output. This might be useful when the graph is going to a
different place than everything else (`-o foo.txt > bar.txt`).

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>