Add support for concurrency limit

[Logicon211]

Currently there is support for the following limits for a function:

const (
    // supported
    LIMIT_VALUE_TIMEOUT     = "timeout"
    LIMIT_VALUE_MEMORY_SIZE = "memorySize"
    LIMIT_VALUE_LOG_SIZE    = "logSize"
    // unsupported
    LIMIT_VALUE_CONCURRENT_ACTIVATIONS = "concurrentActivations"
    LIMIT_VALUE_USER_INVOCATION_RATE   = "userInvocationRate"
    LIMIT_VALUE_CODE_SIZE              = "codeSize"
    LIMIT_VALUE_PARAMETER_SIZE         = "parameterSize"
)

concurrency is a limit that can be manually applied to a function using the wsk CLI, but can't be applied to functions in a wskdeploy manifest.yaml. it is different than concurrentActivations

ie: wsk action update <actionpath> --concurrency 100

It is used to allow a number of functions to be run concurrently in the same container determined by the concurrency number.