Provide multiple sources of opam files

[lefessan]

When multiple projects are built together (typically with git submodules), it would be nice if drom is aware that additional opam files should be taken into account.

Typically, in SuperBOL, we have a Makefile.drom-tpl file that uses a $(MORE_OPAM_FILES) variable to add opam files to be installed.

[lefessan]

A related change in https://github.com/OCamlPro/superbol-studio-oss/blob/master/Makefile.drom-tpl is to add --no-install in:

 	   opam switch create . !{edition} --no-install ;