jepsen: correct jepsen setup/run script

Found issues and resolved:

- Added '$PROJECT_BINARY_DIR/build/' path creation before use.

- Set always ssh-agent restart before add new SSH key, because
  found that in some situations it may fail to add w/o it.

- Found mistake in TF_VAR_id variable setup, inverted ${CI_JOB_ID}
  variable check for it.

Also made improvements in script:

- Added all needed checks for all externally set mandatory variables.

- Added usage message with the list of mandatory and optional variables.

- Added comments to all script steps.

- Added step with checking nodes that they are reachable by SSH.