API support to update password for special user

Provide API support to update password for special user.
This API will be used by OEM Command to update the special
user password - linux uid 0 (root user).
More details can be referred from
https://github.com/openbmc/docs/blob/master/user_management.md#deployment---out-of-factory

Tested:
1. Verified the API with OEM command implementation, and able to
configure the password for user exists in the system /etc/passwd.
2. Verified the negative case for failure

Change-Id: I3c2a7007587e52c7e713f0cd976f249dd84a5f75
Signed-off-by: Richard Marian Thomaiyar <richard.marian.thomaiyar@linux.intel.com>
4 files changed