bugfix: fix eNB name in configuration files

'_' is not a valid character.

S1AP specs say:

    ENBname ::= PrintableString (SIZE (1..150,...))

And X.680 (08/2015) 41.4 table 10 lists the valid characters for
PrintableString. '_' is not there. We replace by '-' which is there.

Connection of eNB to ppenair-cn has not been tested. There is no reason
why it would fail.