HockeyApp plugin should set HOCKEYAPP_INSTALL_URL environment variable

It's convenient to send an email from jenkins with HockeyApp install
link. Especially if there are multiple uploaded products. HockeyApp
itself would send one notification per product.

This patch adds HOCKEYAPP_INSTALL_URL and HOCKEYAPP_INSTALL_URL_n
environment variables after successful upload. The 'n' is replaced
with the app index.