It's possible to add more than one U1 account from system settings on the phone

SRU information:

[Impact] Accidental creation of multiple UbuntuOne accounts can compromise the functionality of U1 (whose stack does not support multiple accounts).
The fix consists in disabling (greying out) the "Ubuntu One" item when an account for it already exists.

[Test Case] With these steps it's possible to create two U1 accounts:
Open **System Settings**.
Go to **Accounts**.
On the **Add Account** section, select **Ubuntu One**.
Enter the email address.
Enter the password.
Click **Continue**.
The **Accounts** screen is shown.
Click **Add account...**
Select **Ubuntu One**.
Enter the email address.
Enter the password.
Click **Continue**.

[Regression Potential] Several packages are involved in this bugfix:
- ubuntuone-credentials: the only changes done here is the addition of a
  "<single-account>true</single-account>" line in the ubuntuone.provider XML file.
  This cannot cause any regression (and older versions of libaccounts-glib simply
  ignore any unrecognized elements).

- libaccounts-glib: added a new API to return the value of the <single-account>
  element; also here there's no risk of regression, as applications now using this
  new API are unaffected. Besides, unit tests have been added to cover this API.

- libaccounts-qt: this is just a thin wrapper around libaccounts-glib. So the same
  API was added here, with unit tests. Other code paths are unaffected by this
  change.

- accounts-qml-module: this is a wrapper for libaccounts-qt, to provide a QML API.
  The value of the <single-account> element has been added as a role in the
  ProvidersModel class. Again, this doesn't affect the existing functionality, is
  well covered with unit tests and moreover at the current stage the ProvidersModel
  class is known to be used only by the ubuntu-system-settings-online-accounts
  project.

- ubuntu-system-settings-online-accounts: support for reading the <single-account> value
  (via accounts-qml-module) and if that is true, then check if there are already some
  accounts created for the given provider. The code changes consists in about 10 lines
  of code, and all but one are only triggered if the <single-account> flag is true
  (which happens for the U1 provider only). The code has been tested and works.

=====================
Original description:

Open **System Settings**.
Go to **Accounts**.
On the **Add Account** section, select **Ubuntu One**.
Enter the email address.
Enter the password.
Click **Continue**.

The **Accounts** screen is shown.
Click **Add account...**
Select **Ubuntu One**.
Enter the email address.
Enter the password.
Click **Continue**.

After adding the first U1 account, the second time you click Add account, U1 should not be available as an option.

See the attached screenshot.