ec2-api: Disable describing of instances using deleted tags as filter.

Previously, one could describe instances using a deleted tag filter
that was once associated to the instance. The current change disables
describing instances via a deleted tag. Also,minor refactor in the
same function.

fixes bug #1197242

Change-Id: Iac6aa85b69d5a6dc563c00f3497cdab08e178ec1
Add ability to factor in per-instance overheads

Currently free memory calculations in the compute layer use a single
value to account for all memory overhead needed by a host.  This value
is: CONF.reserved_host_memory_mb

This patch adds the ability to factor in per-instance virtualization
overheads into usage calculations.  Virtualization overhead may differ
based on the flavor or other factors such as number of vcpus on a host.
This feature's goal is to provide the ability to do more accurate
computations of a host's overhead without having to pre-configure a
best guess in advance.

The actual calculation of overhead is left up to each virt driver to
implement.  This patch defaults to zero overhead for compatibility.

Change-Id: I4febfa2c837e0f80cee85ca74553829c960a3efc
Merge "Add ability to factor in per-instance overheads"
Merge "ec2-api: Disable describing of instances using deleted tags as filter."