Figure out efficient handling of local packages

[jfischer]

Some of the datablox blox require very large (50MB) packages, including Solr and MongoDB. We don't want to include them in the engage distribution of datablox, since they aren't always used. However, if each node that needs them has to download the package each time, this can take a long time and waste a lot of bandwidth. One solution is to stick the packages on the local node in a well-known location (e.g. ~/sw_packages) and have the install use them instead of downloading the package from the source.

Currently the install_datablox.py script will look in ~/sw_packages and copy any files to <deployment_home>/engage/sw_packages. This works, but it causes problems on the master node, as any packages on the master node will end up in the engage-dist.tar.gz distribution that will be used to install all the other nodes. A better solution might be to have the engage.engine.library module always look in that directory without having to copy the file over to <deployment_home>engage/sw_packages