Add network-manager, ppp, pppoe as dependencies

- Since freedombox-setup itself does not use ppp or pppoe, it is perhaps
  not right to add these two as dependencies to freedombox-setup.

- Add network-manager as dependency although it is already dependency of
  freedombox-setup.  This is so that one day Plinth can perhaps work
  independent of freedombox-setup.

INSTALL

@@ -11,7 +11,10 @@
     libjs-modernizr \
     libjs-bootstrap \
     make \
+    network-manager \
     pandoc \
+    ppp \
+    pppoe \
     python3 \
     python3-augeas \
     python3-bootstrapform \