Add support for load-balanced RStudio setups

[kapeels]

From RStudio requirements,

• User accounts must be accessible from each node and usernames and user ids must be identical on all nodes.

Currently, the linux account UIDs are auto-generated.

Changes needed:

• Calculate/Fetch uid for the authenticated user
• Seed the calculated uid while creating new user accounts

Approach 1 - Requires code changes in ohmage

A new attribute should be added for ohmage users. This attribute could be calculated from within ohmage (perhaps just use the numeric id in the MySQL ohmage.user table).

[kapeels]

Approach 2 - Establish strict ohmage username conventions

Only usernames that are in the form: PREFIX-DIGITS can access RStudio.
Example: lausd-12532, cvuhsd-52312, swhsd-92141

Use a hash function that uses the usernames in above format to generate uid.

[kapeels]

Going with the second approach, a configuration file may be needed to define the allowed prefixes. It could be a JSON array (ordered) with implicit offset generation.

Format:

{
"allowed_prefixes": [ "lausd", "cvuhsd", "swhsd" ],
"offset_multiplier": 1000000
}

The offset may be calculated with the following sample function:

( offset_multiplier, prefix_array_index ) => offset_multiplier * (prefix_array_index + 1)

The result of this function will then be added to the "digits" in the username to generate uid. Since usernames are unique in ohmage, it can be guaranteed that there will be no collisions in the generated uids.

[kapeels]

Approach #1 with minimal changes to ohmage