Make retrieval of vmware_host_pass optional

Before this change, a non-vmware user would (without developer mode
have to create a data bag item for this pass even when not using it,
because otherwise the get_secret's call would fail.

Now, the lookup depends on the value of

    node[openstack][compute][driver].split('.').first

to work with both vmwareapi.VMwareESXDriver and vmwareapi.VMwareVCDriver

Implements: blueprint vmware-password-databag
Change-Id: I8c003deb2181b8120b343f85f8759d5de19036ca
Merge "Make retrieval of vmware_host_pass optional"