k5test should use passed in paths, even when system krb5 is present

[DirectXMan12]

Currently, the _discover_path function in realm.py does the following:

1. checks $PATH with which, return that if present
2. if not found on $PATH, check in the user-specified paths dict, and return that if present
3. if not found in the user-specified paths dict, return some default path (/usr/sbin/xyz or /usr/bin/xyz).

This means that system copies installed on $PATH would always be chosen over manually specified paths in the constructor. Instead, the logic should probably instead be:

1. check in the user-specified paths dict, and return that if present
2. if not found in the user-specified paths dict, check $PATH with which, return that if present
3. if not found on $PATH, return some default value

This way, if a user manually specifies a path for some binary, we use that even if there's a system copy installed on $PATH (note that the default value is necessary since /usr/sbin isn't generally on $PATH for non-root users)

[svmhdvn]

This should be resolved with #31, as is recommended by both Heimdal and MIT krb5. Passing in a custom KRB5CONFIG=/path/to/krb5-config should work.