Specifying environment for sites and containers

With the introduction of containers, we need a better separation between how the user specifies the environment for the site, and the environment for inside the container.

For example, a user might want to specify a PATH for the host to pick up the correct staging tools or specific version of Singularity, and at the same time set a PATH to find some tools inside the container.

One proposed solution is to have the site catalog specify the site environment, and the container tc entry and job to be merged to set the environment for inside the container.