nsupdate_plugin: Exception logging now includes error output from nsupdate command

Bug 1089697
Bugzilla link https://bugzilla.redhat.com/show_bug.cgi?id=1089697
When troubleshooting DNS record addition/removal errors, it is helpful to have the error output from the nsupdate command. Specifically in scenarios where record changes only fail <1% of the time.
Follow rc convention and use Rails.logger to avoid large amounts of text in the exception string
Split duplicate code into seperate method
Pass dns action for logger as a string rather than a boolean
Add a section to inject information about external cartridges
Merge pull request #5473 from jwforres/external_carts_section

Merged by openshift-bot
Corrected cmd to enable correctly returning STDERR from nsupdate as STDOUT
Added fqdn to modify_dns method
Moved Rails.logger.error to before the raise
Merge pull request #5408 from tiwillia/master

Merged by openshift-bot