Fix BMCException handling

The outer control loop in install.py handled exceptions of type
BMCException defined in remoteinstaller.installer.bmc_management.bmctools.

Redefining BMCException for each platform in local scope caused it
no to be properly handled and the running installation would not get
its state change to failure, instead it would remain in 'ongoing' state
indefinetly.

If per-platform excception customization is required, then it should
be done through inheritance from BMCException defined in bmctools.

Signed-off-by: Alexandru Antone <Alexandru.Antone@enea.com>
Change-Id: Ia6a09ee4b1a4b7e02113d75b02deec8ae7a19ae0