Fix exponential regex performance issue

filter_leading_non_json_lines effectively does

re.match(".*\w+=\w+.*", line)

for every line of output. This has abysmal performance in case of large
Base64-encoded data (which ultimately does not match the regex but does
match the .*\w+= part) as returned e.g. by the template module (diffs).

Replacing the match with

re.search("\w=\w", line)

drops the complexity back to linear, and actually usable with large
diffs from the template module (a 150 KB Base64 diff kept Ansible
spinning at 100% cpu for minutes).

Also, check the easy cases (line.startswith) first while we're here.

Closes: #8932
Update vault docs to indicate the executable script option is part of Ansible 1.7.
Adding test Coverage for mysql_variable module

Use mysql_variable to query, set and update variables.
Assert using user and password to query, set and update  variables.
Assert using single quotes, double quotes and no quotes  when using variables
Add support for environment variables in GCE module.
"allow_no_value=True" for ini_file module so /etc/my.cnf can be read without a problem
Update our search code for the doc site.
Envirionment variable support in GCE inventory plugin.
Update version.
Merge pull request #8397 from lukesneeringer/gce-envvars

Add support for environment variables in GCE module.
Merge pull request #8398 from wrosario/integration_mysql_variables

Adding test Coverage for mysql_variable module
Merge pull request #8400 from nicocesar/devel

"allow_no_value=True" for ini_file module
Merge pull request #8405 from wenottingham/patch-2

Update version.
Fixes issue #8339 where Scientific Linux incorrectly tries to use FedoraStrategy of hostnamectl
Fix a typo in gem module documentation
Add version_added attribute for regexp parameter.
Include unicode and var_precedence tests in 'all' target
Merge branch 'bug8392' of git://github.com/gnosek/ansible into devel
Add some whitespace to make paramiko_ssh more browseable
Use atomic move function on known host file in paramiko to prevent rare occurance of Control-C
during file operations.
Remove unnecessary unicode conversions from parse_kv

Fixes #8425