Name the securitygrouprules.direction enum.

Updates to the SecurityGroupRule model and migration so that we
explicitly name the securitygrouprules.direction enum. This fixes
'Postgresql ENUM type requires a name.' errors.

Fixes LP Bug #1099267.

Change-Id: Ia46fe8d4b0793caaabbfc71b7fa5f0cbb8c6d24b
Merge "Name the securitygrouprules.direction enum."