CLI: use ENV to determine RPC type

- Take RPC type from environmental variable PERUN_RPC_TYPE.
- Value must match AuthName property of Apache sites configuration
  for URL specified by PERUN_URL environmental variable.
- If not set, "Perun RPC" is used as default, which matches our
  basic auth used for service components on virtual appliances.

Before putting this script on devel/production instance, please set
this ENV in start_engine.sh script to "Kerberos META" value.

- Updated readme.