stash.sh doesn't clear the cache under $*_AUTH_CACHE_DIR

if [[ "$KEYSTONE_TOKEN_FORMAT" == "PKI" ]]; then $*_AUTH_CACHE_DIR is used to cache ca.pem and signing_cert.pem for
the keystone server.

If we run ./stack.sh again, the ca.pem and signing_cert.pem are regenerated. And the cache become invalid.
Command like 'nova list' would make nova-api use the invalid pem files to verify CMS. As a result:
Verification failure
140630604306080:error:0407006A:rsa routines:RSA_padding_check_PKCS1_type_1:block type is not 01:rsa_pk1.c:100:
140630604306080:error:04067072:rsa routines:RSA_EAY_PUBLIC_DECRYPT:padding check failed:rsa_eay.c:721:
140630604306080:error:2E09A09E:CMS routines:CMS_SignerInfo_verify_content:verification failure:cms_sd.c:900:
140630604306080:error:2E09D06D:CMS routines:CMS_verify:content verify error:cms_smime.c:425: