Support changing instance admin password in Horizon

An instance admin password can be change from CLI using:
$ nova set-password <instance>

Supporting changing an instance admin password in Horizon:

In the instances list page, clicking 'More' for an instance, displays the dropdown with available actions.
Add another action next to 'Retrieve Password' action, visible anytime the 'Retrieve Password' action is visible.
The action must be disabled by default in 'defaults.py', similar to 'Retrieve Password' action.

Clicking the 'Change Password' action opens a modal with 2 inputs: new password and confirm new password.

After confirmation, nova client's 'change_password' can be called with the instance id and the new password.