RHEVM tests: have VCR match only on path and query, not full URI
RHEVM tests: do not record username/password/hostname with VCR
* server/tests/test_helper.rb: simplify defaulting of user/passeword/provider
RHEVM unit tests: allow triggering of rerecording with env var VCR_RECORD

Setting VCR_RECORD=all will rerecord the fixtures
* server/tests/test_helper.rb: allow accessing preferred items from config
* server/tests/test_helper.rb: save/load scrubbed config data for each driver

Anonymize user/password and give tests a chance to modify other things
before writing.

For some tests, like RHEV-M, the provider URL contains sensitive
information, like the hostname, which should not be recorded. At the same
time, we need to preserve the URL structure so that VCR matches requests on
playback.
Revamp RHEV-M driver tests and rerecord them

This patch changes the tests so that they do not depend on any hardcoded
values; rather, all these values are loaded from ~/.deltacloud/config

  * store preferences in fixtures/preferences.yml when recording
  * load them from there when not recording
  * use preference items in tests rather than hardcoded ones
  * use driver/credentials from config everywhere
  * avoid a bunch of pointless HTTP requests