Allows for specifying compute node RAM

Allows for specifying compute node RAM

Currently we allow specifying ram per Overcloud VM.  If the node is
detected as ODL, we bump the RAM to a minimum of 10GB.  There is a need
to be able to specify the RAM per compute node in cases where we need
controller RAM to be high, but want compute nodes to be low (like CSIT)
- in order to keep the memory footprint as small as possible.

Changes Include:
 - Adds '--virtual-compute-ram' argument that will override the
   '--virtual-ram' param for compute nodes
 - Fixes a bug where if ODL is used and RAM is overridden to 10GB for
   Control nodes, it was also accidentally being set for compute nodes
 - Modifies '--virtual-ram' to be '--virtual-default-ram' in order to
   clarify this parameter sets the default amount of RAM for all
   overcloud nodes which may be overridden by a role specific arg

Change-Id: Ia36082aa2167d9897f3ec6753d08804352301c63
Signed-off-by: Tim Rozet <trozet@redhat.com>