Fix EXTRA_OPTS handling

In the conversion away from add_nova_opt the EXTRA_OPTS handling
inadvertently replaced all '=' chars in the value rather than
just the first.  Additional '=' is legal for an option value.

FWIW here is the setting that tripped it:
EXTRA_OPTS=default_log_levels=sqlalchemy=WARN,boto=WARN,eventlet.wsgi.server=WARN

Change-Id: I2deb139171250eb0ef5028bb924569cec31e1a4e
Merge "Fix EXTRA_OPTS handling"