CA-109273: [DELL-194] Throws exception if Download & Install used on host in maintenance mode.

1. Added additional check to DownloadAndInstall method in ManageUpdatesDialog.cs: will proceed to the prechecks page in the Install Update Wizard only when there are selected servers on the previous page.
2. Modified code to allow hotfix installation when the host is in maintenance mode; however the host cannot be upgraded when in maintenance mode.
3. Modified test to reflect changes described above.

Signed-off-by: Adrian Jachacy <Adrian.Jachacy@citrix.com>