Fix missing exception catch in is_admin_available()

This commit adds a missing exception catch in the is_admin_available()
method. When non-locking test accounts are used and no admin creds are
available an InvalidConfiguration exception is raised instead of the
NotImplemented exception raised by the locking test accounts provider.

Partial-Bug: #1296955
Change-Id: Ic7006f6a40ac174e4b6793096fb92957df350d61
1 file changed