More exact section matching for tox showconfig

With the original verbose output support in change
Iafeb88eaf9a596603ad4d2134a4574345d5189ab we looked for lines from
tox --showconfig output starting with an opening '[' but verbosity
also causes output from pip install activity to be included if a
tox.requires entry causes it to install packages before continuing
and these lines are prefixed by a process ID number wrapped in
brackets with the command string after that, which prematurely
triggered our search for the start of the INI content. Add a
stipulation that the first INI output line also end in ']' in order
to skip over those additional prepended lines.

Change-Id: If29e5a9abe3b92a145d87f5efc1b93350ea3908a

roles/tox/library/tox_install_sibling_packages.py

@@ -319,7 +319,7 @@ def main():
             if not discard:
                 # Normal operation, tested first for efficiency
                 tox_clean_config += line
-            elif line.startswith('['):
+            elif line.startswith('[') and line.rstrip().endswith(']'):
                 # Once we see a section heading, stop discarding
                 discard = False
                 tox_clean_config += line