DataSourceAzure: update password for the default user if the user exists

[anhvoms]

Proposed Commit Message

DataSourceAzure: update password for the default user if the user exists

cc_set_password will only update the password for the default user if
cfg['password'] is set. The existing code of datasource Azure will fail
to update the default user's password because it does not set that
metadata. If the default user doesn't exist in the image, the current
code works fine because the password is set during user create and
not in cc_set_password

Additional Context

Test Steps

• Deploy the latest bionic image on Azure with password
• Prepare an image but do not delete the user (waagent -deprovision)
• Deploy a new VM from the prepared image with a new password
• Observe that the password is not set appropriately (and that the
  previous password continued to work)
• Apply this patch and go through the steps again and verify
  that new password is set.

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[blackboxsw]

Thanks @anhvoms.

No backwards incompatible changes here. Only adds a top-level password value to the base config, which could be overridden if users provided a "password" themselves in #cloud-config as well.

I reproduced failure case as mentioned by "capturing" a VM under test with password provided and launching new vms from that image. When applying your changeset I confirmed new password was honored and old was no longer valid.