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.

Plus, it's more in line with the other drivers.

Implements: blueprint vmware-password-databag
Change-Id: I48a6e23d6aa9abd106e65e70b40662c8d4cca5fc
Merge "Make retrieval of vmware_host_pass optional"