Export all TMT_ROLE_* envvars to all phases

[happz]

A follow-up for #1790: patch constructs and exposes TMT_ROLE_* when running a phase. But:

• queue task may see only a subset of all guests, therefore envvars might not list all existing guests for a given role, or miss some roles completely.
• there is a similar functionality in multihost plugin, but that one depends on multihost metadata, therefore does not consume the role and name of provisioned guests.

Neither of these two blocks involved in the TMT_ROLE_* generation seems to be good enough, we need a proper code to expose all known roles and all their corresponding guests.

There are also envvars for all guests (e.g. TMT_GUESTS) and all guests in the current phase.

[happz]

Implemented in #2072