[Sahara] Support Nova availability zones

Add an optional field in the node group template page.  It is used for
selecting the availability zone where to spawn instances.

Change-Id: Id504ec3d0d8a398f79cc45568169e67f54a6a9b8
Implements: blueprint support-nova-availability-zones
add Column attribute for help tooltip in heading

To use: add attribute help_text=_("foo") to tables.Column(...)
This will add a help icon next to the Column name which
you can hover-over to get informational text.

Change-Id: I10a28bd51efbaf83c617b30b31d861445418f9d8
Closes-Bug: #1381633
Merge "[Sahara] Support Nova availability zones"
Merge "add Column attribute for help tooltip in heading"
Merge branch 'master' of https://github.com/openstack/horizon