Clean up handling of Legion config options

[manopapad]

• Always build Legion with python support and FFI bindings. This way even if a user starts with a default cmake build, then does pip install, the Legion build that cmake did will automatically be compatible with the python layer. We could just change the default to be ON everywhere, but I don't see a reason why we need to support Legion builds w/o python and bindings, but maybe folks have a different opinion.
• Move unconditional Legion configuration flags out of install.py, and leave a note to put those in the cmake configuration. As is we run the risk that developers will only update install.py and forget to update the cmake-only defaults.
• Document major required Legion configuration flags, so that users know how to build Legion such that it's compatible with Legate. Automatically check all the ones that Legion exports (others will have to wait until the Legion cmake build starts exporting them).

[jjwilke]

LGTM. Installation works and checks correctly catch errors.

[trxcllnt]

This makes it impossible to build the legate C++ library without Legion built w/ Python bindings. Is this really what we want?

[manopapad]

This change is my attempt at avoiding the following fell-bad scenario:

• user builds Legion separately, forgets to add python support
• user builds legate.core C++ bits using cmake directly
• users does pip install
• user realizes nothing is working, because they built Legion w/o Python support (and the build process never complained)

I would also be ok with a situation where the cmake build doesn't enforce Legion_USE_Python, and instead that becomes necessary only when you try to install the python pieces of legate.core. Do you think that's possible/preferable? Or maybe you have a different approach to avoid this scenario?