Be explicit about text file encoding

Be explicit about text file encoding

Python PEP 597 (https://www.python.org/dev/peps/pep-0597) recommends to use an
explicit encoding for text files instead of the default locale encoding.
Pylint 2.10 adds a new checker named unspecified-encoding for that.

The present patch adds explicit utf-8 encoding to open() calls in nfvbench and
fixes pylint unspecified-encoding warnings.

Remark: this patch does not change nfvbench behaviour on systems where utf-8 is
the locale encoding, which is generally the case on Linux systems.

Change-Id: Ic4dfb37e1ea958452a0173f7630a68f0d95071ae
Signed-off-by: Gwenael Lambrouin <gwenael.lambrouin@orange.com>