installer: Import shutil to fix --overwrite
installer: Import shutil to fix --overwrite

I tried to copy the logic from imagefactory.py for this, but
omitted the import.

maxamillion/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox