[config/pe_bootstrap] Normalize role value

Symbols are preferred over strings when referencing hash keys and
discrete string-ish values in Ruby, but they're a major pain to
specify in configuration files. For instance, the syntax for specifying
symbols in YAML is `!ruby/sym role`, which is very clumsy.

Instead of forcing users to handle this little idiosyncrasy, this commit
interns the `role` value when specified so that end users don't have to
know about this, and consuming code doesn't have to normalize strings
and symbols.
Remove deprecated `step` configuration

The ability to process steps as independent commands dates back to when
pe_build was part of a monolithic plugin that disallowed use of other
provisioners. Since that limitation was removed this is no longer
needed and can be removed.
Merge branch 'normalize-role'
Merge branch 'remove-steps'