rgw: improve convenience for key operate.

rgw: improve convenience for key operate.

1.key-type assignments based on context if it wasn't specified
In user    operate context, key-type assignment to KEY_TYPE_S3
In subuser operate context, key-type assignment to KEY_TYPE_SWIFT
In key     operate context, key-type assignment based on user type

2.fix RGWSubUserPool::add()
When create subuser generate secret by default

3.fix RGWAccessKeyPool::generate_key()
Avoid wrong key's username when create user and subuser at the same time
Check empty secret

Signed-off-by: Ce Gu <guce@h3c.com>