ureport: fall back to the hardcoded rhsm cert dir

This commit adds a fallback execution path for cases where getting the
rhsm entitlement dir path via an execution of python from the
librepor-web library fails for any reason.

See commit 2b20c9f91342da7744ae40ee623735ab95f83219
Related #1140224

Signed-off-by: Jakub Filak <jfilak@redhat.com>