Add some hooks, searching in GVM_DIR/hooks

E.g. for grails 2.1.0 the system looks for a post-install.sh in

hooks/grails/2.1.0 hooks/grails/2.1 hooks/grails/2 hooks/grails/default hooks

It executes the first one it finds and then exits the loop.

N.B. this patch does not install any hooks.  We need to decide how that could
be done in a data-driven way.  Registering a hooks.zip per candidate in Mongo
might be one option, and unzipping it during an installation.