Safer check for enabled in trusts

For some LDAP backends, the enable attribute is not
directly set.  Since we have a wrapper function in identity/core,
we can use that instead.

Adds tests that disabling trustor or trustee prevents
trust execution.

Closes-Bug: 1367952

Change-Id: I6d3c26e0e6e6c0d27ea7b910340fae5792eaceda
Merge "Safer check for enabled in trusts"