* Set allowed OIDs to any value when generaing a CA.

Also, allow utf-8 in addition to utf8 as the OID type specifier, and
allow `*` to specify any OID of a supported type.

* Update PKI docs

Running 'vault auth' with no parameters was panicking:

panic: assignment to entry in nil map
	github.com/hashicorp/vault/command/login.go:255 +0xdee

Now it will show help.

sha_256 is supported but not referenced in our API docs.

When Vault is concurrently creating and revoking leases for MongoDB
users as part of the database secrets engine, and then loses connection
to MongoDB, it can panic. This occurrs because the RevokeUser path does
_not_ lock the mutex, but the CreateUser path does. Both threads of
execution can concurently decide to call c.session.Close() in
mongodb/connection_producer.go:119, and then mgo panics when the second
close attempt occurs.

* Set allowed OIDs to any value when generaing a CA.

Also, allow utf-8 in addition to utf8 as the OID type specifier, and
allow `*` to specify any OID of a supported type.

* Update PKI docs

Running 'vault auth' with no parameters was panicking:

panic: assignment to entry in nil map
	github.com/hashicorp/vault/command/login.go:255 +0xdee

Now it will show help.