(maint) Don't strip whitespace from yumrepo keys

Yumrepo values may have leading or trailing whitespace, and stripping
this can cause resources to flap. Since it's not strictly necessary and
the yumrepo code in 3.4.3 didn't handle this, we can omit this for now.
(PUP-2179) Add test coverage for IniConfig::Section
(PUP-2179) Extract standalone INI file class
(PUP-2179) Implement INI single file formatting
(PUP-2179) implement storing for single INI files
(PUP-2179) Implement simple INI file collection
(PUP-2179) Replace Puppet::Util::IniConfig::File
(PUP-2181) Delete ini file entries on absent property