Add support to configure the number of browser processes per instance.

[adam-hampton-sp]

Modifies AwsVmManager in the following ways:

• CHROME_THREAD_COUNT becomes non-final.
• FIREFOX_IE_THREAD_COUNT becomes non final.

New values are applied at runtime if the awsProperties contain keys like:

• node_max_processes_chrome=8
• node_max_processes_firefox=1

[mhardin]

@adam-hampton-sp are you able to add unit tests around the new logic you added?