Fix modified (accidently) variable name in keystore.py
Merge branch 'pbkdf2-iterations-vary', remote-tracking branch 'origin' into log
Add LOGLEVEL_SCHEMA to tuf.formats

log.py previously checked the format of log level arguments with LENGTH_SCHEMA.  LOGLEVEL_SCHEMA might be clearer.
Update comments and schema type checked

Expand the comments on 'logging.Formatter.converter' to discuss the converter attribute.  Modify LENGTH_SCHEMA to LOGLEVEL_SCHEMA.