allow usage of variables in bash-completion

[Karl-Philipp Richter]

I'd be nice if `bash-completion` would recognize that in `export M2_HOME=$HOME/apache-` it's desired to display auto-completion of names starting with `apache-` in the home directory.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: bash-completion 1:2.1-4.3ubuntu1
ProcVersionSignature: Ubuntu 4.10.0-22.24-generic 4.10.15
Uname: Linux 4.10.0-22-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.4-0ubuntu4.1
Architecture: amd64
CurrentDesktop: Unity:Unity7
Date: Sun Jun 18 22:22:24 2017
InstallationDate: Installed on 2015-12-12 (554 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
PackageArchitecture: all
SourcePackage: bash-completion
UpgradeStatus: Upgraded to zesty on 2017-05-01 (48 days ago)