Make 'create docker VM' button always enabled

Before this change, the button was enabled only when none of the VMs
were selected, which is the case only when the grid is opened.
It should have the same behavior as 'create VM' button which is always
enabled.

Change-Id: I1d2b3e6f6d05cfdbaf93f0694dbe6ab13f3aeaf1
Signed-off-by: Alissa Bonas <abonas@redhat.com>