allow bash completion for overwriting already defined environment variables

[Karl-Philipp Richter]

If an environment variable (e.g. `JAVA_HOME`) is set, there's no disadvantage in recognizing the variable during definitions in the form of `env JAVA_HOME=` and allow path completion for the following part/the variable value.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: bash-completion 1:2.1-4ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-11.11-generic 3.19.3
Uname: Linux 3.19.0-11-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Apr 4 15:30:55 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-02 (1 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Beta amd64 (20150326)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: bash-completion
UpgradeStatus: No upgrade log present (probably fresh install)