add install script into package

[sdesbure]

current release package doesn't provide script/install_plugins.shwhich is mandatory if we want to install from package. this commit is adding them, allowing to install helm push in a "non Internet" environment (with the use of HELM_PUSH_PLUGIN_NO_INSTALL_HOOK)

[mparada]

I'm not sure it is the expected solution. But I found a way to install the plugin with a local file without using the helm plugin install command: https://github.com/chartmuseum/helm-push/issues/161