Add config option to limit image members

This patch adds the image_member_quota config option. This allows a deployer
to limit the number of image members allowed per image. The default value
is 128, to be consistent with other quota defaults. Users will only be able
to update an image if the result of the transaction would be under this limit.

This is for both Glance v1 and v2

Fixes bug 1252459
docImpact

Change-Id: I02f5e82ca4c4acf6cd7bc94f9b99086054a616c9