Allow defaultConfigOverwrite to replace httpd.conf

[gibizer]

For nova-api and nova-metadata we now allow to replace the whole
apache httpd.conf file. The replacement can be a template the same
way as the default file in templates/novaapi/config/httpd.conf is a
template.

This is expected to be only used for advanced troubleshooting and not
for customization for production use. Any permanent customization needs
to go through the CR interface (and potentially need an RFE to extend
such interface)

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[gibizer]

Measurements
Same setup as in PR #1074 but here instead of 3 nova-api pods with 2 procs each we overwrite the httpd.conf and used a single nova-api pod with 6 procs instead. Overall we had the same amount of WSGI procs.

load	result PR 1074	result with this PR
c25, t10	2.3 req/sec, no kill	2.45 req/sec, no kill
c50, t20	2.21 req/sec, no kill	2.29 req/sec, no kill
c75, t20	2.13 req/sec, no kill, 30 req error	1.52 req/sec, no kill, 75 req error
c100, t20	2.33 req/sec, galera kill, 360 req error	2.66 req/sec, galera kill, 855 req error (~50%)