We couldn't use environment variables safely to configure storagenode, since we introduced the embedded updater.
For example STORJ_DEBUG_ADDR=localhost:11111 would try to set debug port 11111 for both the storagenode and storagenode-updated, causing port conflict.
This small change enables to configure storagenode-updater with STORJUPDATER_... environment variables.
Tested with creating custom image and installing to my own storage node.
Change-Id: I6b0a601a4dc63d2d1ff3c191ae89981434e55c30