[WIP] Fix, update and refactor configuration script.

### Motivation
Our *nix configuration script is somewhat messy. Dependency versions have fallen out of date, especially on Fedora. Proper sandboxing via `virtualenv` is not enforced across installations. This PR is an attempt to fix some of the problems and factor repetitive code.  

### Changelog
* Factor creation of `virtualenv`.
* Use `virtualenv` on Gentoo.
* Do not parrot executed commands to output.
* Sanitize Fedora configuration: upgrade `bitcoin-release` to version `1.6`; prune unneeded dependencies
* Minor fixes and grooming

### TODO
* Properly set `LD_LIBRARY_PATH` when executing OB in Fedora. This is not strictly related to this PR, but it is required to test it. (see: #990)