added $server_root parameter

[ptomulik]

Previously, the $httpd_dir was used as ServerRoot in httpd.conf
template. On some installations httpd_dir is not same as ServerRoot,
for example on FreeBSD apache22 (installed via ports) has ServerRoot set
to /usr/local by default.

This PR adds $server_root parameter to apache::params. Its purpose is to
be substituted in httpd.conf as ServerRoot. This parameter is used
in #342. The purpose of the whole thing is to plit #342 into smaller
parts to make review process easier, see
#342 (comment)

[apenney]

Do you think this is worth exposing as a parameter? Right now it's just in params.pp and not as an actual param. I don't know if people would generally have cause to change this, but you never know.

[ptomulik]

I don't know, I've just organized this parameter simillary to $httpd_dir (which was used previously as ServerRoot). I would prefer to leave this as is (less work for me). I think having additional parameter in apache class doesn't hurt (does it?).

[apenney]

Maybe I'm losing my mind but when I look at the diff the problem is that there isn't a parameter in the apache class, there's just an entry in params.pp and no entry in init.pp to match, which is what I meant. I'm fine with it being an additional parameter, which is what I want, it just isn't :) It just needs one extra line :D

[ptomulik]

OMG, you're right! It must be a result of wild rebasing. It's corrected now.