UCT/IB: add UCX_IB_PRIORITY_SLS env configuration

[roiedanino]

What

New env configuration - UCX_IB_PRIORITY_SLS - a list separated by comma listing SLS for each priority, priority is indicated by index, meaning first SL to be used as lowest priority (0) and last SL as highest priority.

Why ?

As part of the upcoming new feature of setting priority for active messages, we would want to map priorities to SLs (service levels) in IB transports

[yosefe]

@roiedanino pls squash