Add feature to let users update their own password

[gctucker]

The idea here is to be able to let a admin user create new user accounts with a random password, then let the actual user update the password with a real one. This is an interim solution until we enable public sign-up for any user, although even then it will be necessary for users to update their passwords anyway.

I think this boils down to this:

• API endpoint created to update the user's password (e.g. POST to /password
• method added to kernelci.api base class to set the current user's password
• command added to kci user to make use of the API feature