Fix conversion to JS for HTML reports

Some new fields in InfluxDB have plain text contents that AST cannot
parse as valid expressions, resulting in exceptions being raised by
yardstick CLI commands "report generate" and "report generate-nsb".

Reworked _generate_common() to properly handle and convert:
- unicode, str and missing keys
- None, unicode, str, float, long and int values
- float, long and int stored as unicode or str values

Added test__generate_common() unit test, to verify all possible
conversion flavors.

Also renamed incorrect "tasks" to "metrics".

Also fixed warning in Python3 for some unit tests, caused by
deprecation of unittest.TestCase.assertRaisesRegexp().

JIRA: YARDSTICK-1367
Topic: report/html_table (10 of 12)

Change-Id: Iff75bfd2c1dcaf3806f67f52f0ea594f10aceb5b
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>