Add ExecutionSystem contexts

[bluekeyes]

There are many situations when you want all commands associated with a specific execution system to use the same default working directory, environment, prefix (sudo -u user, bash -c), or other properties. Ideally, this will be a provider-type interface that can compute values dynamically (e.g. refresh the environment by executing bash -l -c env). This leads to some complications with recursive execution, but they should be solvable.

This might relate to a larger issue of better exposing the various options that can be passed to systems when they are constructed.

[bluekeyes]

Original requested to better support automating our deployment service that modifies the user's .bashrc file when installing Postgres.