Add RHEL 7.x latest image for Azure
Add RHEL 7.x latest image for Azure

This patch adds the official Azure RHEL "7-RAW" image to the default
list of VM images in Azure plugin. Also switched the default RHEL
image used for testing to the new "7-RAW" RHEL image.

NOTE:
RHEL seems to have changed their VM image SKU naming convention with
7.4. Previously (before 7.4) the RHEL images have specific SKU with
minor relese verinos (i.e. 7.2). For RHEL 7.4 deployed from Azure
portal, the sku has become "7-RAW".

Testing Done:
- Passed all UT.
- Passed the following live tests (that deploys a RHEL image):
  vmAllocateWithImageThatHasNoPurchasePlanExpectSuccess
  vmssAllocateWithImageThatHasNoPurchasePlanExpectSuccess

(cherry picked from commit b97fff7681a1b3e960860c9ec9716c2e462d5684)

TobyHFerguson/director-scripts,cloudera/director-scripts,cloudera/director-scripts,TobyHFerguson/director-scripts