added option for partial deployment specifications

[rmm-fcul]

• if arena file specifies more casus (or even layers) -- such as a full physical setup -- than a dep file does, this new option allows the creation of deployment files for the subset specified.
• however, if the option is not explicitly set by user, the mismatch is caught and an exception is raised
• resolves #14
• a partially generated fabfile requires no modification of assisirun to be run on the targets successfully
• tested in simulation with partial and complete specifications, with option on & off.

[rmm-fcul]

Note: the fixes for #72, #14 and #43 are all made as branches from master @commit 5db6694. They modify same/similar parts of the files and didn't merge automatically if applied sequentially; I did a manual merge here: https://github.com/rmm-fcul/assisi-python/commit/b87a95dcd9a6fa97f798b31efdf6a24c80ae1a81