core: Use images instead of volumes at CDA message

The following message might be misleading since it uses the word
"volume", which could be related to gluster: "Cannot remove Data Center while there are volumes on it.
-Please remove all VMs and Templates first."

There is also no need to indicate Vms or Templates since there could be
also floating disks.

Rephrasing the message to be "Cannot remove Data Center while in contains
Disks. Please remove them first."

Change-Id: I2cf86b6f968c4a6ff7178b742eb57077543d6143
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Bug-Url:https://bugzilla.redhat.com/955557
core: Add CDA message when domain is inactive

When creating a VM from template and one of the domains sent by the user
is inactive.
engine should add a validation message for the user that the storage is
not active and fail the operation gracefully.

Change-Id: Ie443ccc36b2e7ccbf889f52e2b4c661269bc9aa5
Bug-Url:https://bugzilla.redhat.com/975053
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
core: Change warning message when import VM

changing warning message to be more clear when importing a VM which
based on a template.

Change-Id: I01300139bca2a278331f56bef7f7d17593f884d5
Bug-Url:https://bugzilla.redhat.com/964647
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>