TT#44161 Support Debian/buster by Build-Depending on either npm or nodejs

Debian/buster as of 2018-09-07 provides npm v5.8.0+ds-2.
Fall back to nodejs if npm isn't available (e.g.
on Debian/stretch with nodejs in our own repositories).

Change-Id: I27c259d6a671aea47a972d39d1f5cdec1f26ff10

debian/control

@@ -6,7 +6,7 @@ Homepage: https://www.sipwise.com/
 Standards-Version: 3.9.8
 Build-Depends:
  debhelper (>= 10),
- nodejs (>= 4.6.2~),
+ npm | nodejs (>= 4.6.2~),
 
 Package: ngcp-csc-ui
 Architecture: all