Set rmcp session privilege level in session activation

[canteuni]

The issue

After PR #144 was merged, I tried to connect to a BMC using an "OPERATOR" user with interface "rmcp", but I kept getting error messages such as :

CompletionCodeError cc=0x86 desc=requested maximum privilege level exceeds user and/or channel privilege limit

It works perfectly with interface "ipmitool".

The cause

I think this error comes from the _activate_session method in pyipmi/interfaces/rmcp.py, which uses a hardcoded Session.PRIV_LEVEL_ADMINISTRATOR by default. If the BMC user only has "OPERATOR" privilege for example, trying to establish a session with maximum privilege requested set to "ADMINISTRATOR" it will result in an "Insufficient privilege" IPMI error.

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I have also added an example of set_priv_level usage in the README and in the examples/ folder to show how it is supposed to be used.

[coveralls]

coverage: 69.565%. remained the same
when pulling 689bf97 on canteuni:set_rmcp_session_privilege_level
into 3e018e3 on kontron:master.

[canteuni]

Hi @hthiery , will you have the time to look at this ? No pressure of course, I can provide additional details if needed.

[hthiery]

Looks reasonable .. I merged it

[hthiery]

thanks