webadmin, backend: control of hosted engine maintenance mode

Allow control of hosted engine local/global maintenance.  Global
maintenance is performed via right-clicking on the host, while local
maintenance is tied into the existing host vds maintenance operation.

Change-Id: I0f76f7ad63bcf6d7871c362b46cfa6e928eb9c74
Bug-Url: https://bugzilla.redhat.com/1047649
Signed-off-by: Greg Padgett <gpadgett@redhat.com>