Werk #397: mrpe: tolerate performance variable names with spaces

Some bizarre legacy check plugins use spaces in the names of their performance variables. Nagios allows this under certain circumstances. The mrpe check would fail with an exception, though.

The fix now tolerates spaces, but uses just the last component of the variable name in that case. All RRD graphs will be created correctly, though with an incomplete name. In most cases that should be OK.

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