Admin password should not be stored in clear text by default

[4a6f656c]

The Jenkins charm currently writes out the Jenkins admin password to a ~jenkins/.admin_password file, which results in the password being store in clear text. This poses a number of risks for the password being leaked/exposed - this should not be enabled by default (if supported at all).

[mthaddon]

We should remove the 'username' and 'password' config items as well. Instead there should be a juju action to retrieve these credentials, and they should be auto-generated by the charm.