Add flag to make IsolatedHostsFilter less restrictive

This patch allows to run 'isolated' and 'non isolated'
images on isolated hosts by adding a flag
'restrict_isolated_hosts_to_isolated_images'.

If the flag is set to True then 'non isolated' images
can't be run on isolated hosts (same behaviour as the
current filter) otherwise they can be run on isolated
hosts.

DocImpact

blueprint improve-isolatedhostsfilter

Change-Id: I8c092caf32c05be88a547a7e8cb0530cc4925080
Merge "Add flag to make IsolatedHostsFilter less restrictive"