testsuite: install (most) optional dependencies correctly for travis-ci

Still can't get the python selinux libs to install
schemas: fixed augeasCommands group in Path entries
Encryption: fixed unit tests
POSIX: skip loading POSIX sub-tools that raise ImportError

This mimics the behavior for "real" tools
Augeas: Only install unverified commands
Augeas: Added docstrings, fixed some minor pylint issues