Adds more policy control to cells ext

Allow create,delete,update and sync_instances to
have fine grained policy control.

UpgradeImpact
DocImpact

Change-Id: I2e32ae2f37d3b599585d25535c79eecf6485b462
Closes-Bug: 1335901
VMware: do not cache image when root_gb is 0

Save space on datastore when the instance size has root_gb of
0. There is no need to cache the disk as it already exists on
the datastore.

TrivialFix

Change-Id: Iac75d97f40a1a91b8fa2df7fb4318fc2965fd93a
Closes-bug: #1304593
Convert to importutils

We had some remaining users of __import__ that really deserved to
be converted to oslo's importutils.

Change-Id: I8c0c3e885151018b974025087a4abc96dd6ae77c
Merge "Convert to importutils"
Merge "Adds more policy control to cells ext"
Merge "VMware: do not cache image when root_gb is 0"