hostnamectl: respect SYSTEMD_COLORS

hostnamectl: respect SYSTEMD_COLORS

Instead of the wrapper functions the color defines where used directly.
This was especially a problem for ANSI_HIGHLIGHT_YELLOW which is not
legible at all for light themes such as solarized. The 16 color palette
fallback can be set to a sane yellow or orange on that matter but it was
not used though `SYSTEMD_COLORS=16` was set.

Use the proper wrapper functions and let the right color be selected the
way other components do it already.