Address comments from reviews

* in #Pkg::Config::issue_reassignments
 - Addressed a bug where we assumed the return from an assignment was a
   sufficient judge of if a variable was set. If the variable was set
   intentionally to 'false' this will return unexpected results. Instead, check
   explicitly for 'nil.'

* in Pkg::Config::Params
 - Fixed up tabbing for hashes to be consistent

* in Pkg::Tar::tar
 - Made sure that we actually fail if we can't create a tarball, and warn the
   user about the flags we require of the 'tar' command.

* in Pkg::Util::Tool
 - is_git_repo? is more idiomatic than is_git_repo. To keep it backwards
   compatible, define the new one, but make an alias to the old one.

* in Pkg::Util::Serialization
 - In the spirite of future compatibility with other serialization formats, we
   should lazy load 'yaml' when we need, it not when we load the class

* in Pkg::Config
 - Remove unused #symbolize_hash method. This was left behind by a previous set
   of work that isn't in place anymore.

* Whitespace fixes

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Address review comments in tar.rb

* In Pkg::Tar
 - Rename variable (t) to (template) for easy reading
 - Rename variable (rel_path_to_erb) to (rel_path_to_template) for easy reading
 - use 'cd' instead of 'Dir.chdir' since we've included fileutils. It's just cleaner.
 - Wrap all paths in our shell execution of `tar` in single quotes for safety

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>