Uniform use of posix path separation in runtest module.

Erlang modules expect paths in posix format where path
separation is '/'. This is the case even on Windows.
Since posix path work fine on Windows platform we can
force posix path by replacing '\' with '/'.

Cmake module option -m uses combination of absolute paths
and relative path. Forcing posix path format by string
concatination caters only to relative paths at runtime
Absolute paths will not work as expected with string
concatination. OS module specifically os.path.join will
handle these cases transparently. For example string
concatenation of /A/B/ and /A/B/C would give /A/B/A/B/C
where as os.path.join will result in /A/B/C which is
what is expected.

Since paths emitted by cmake would be used by other
modules, we need to use os.path module to generate
correct paths at runtime in our script and force
posix path format by replacing Windows style path
separation symbol '\' by Unix style path separation
symbol '/'. Also ensure that space escaped by '\'
are preserved, since directory names might contain
spaces in them. Hence regular expression substituion
is used.

Change-Id: I63cf5bd4b58fe7a0c78abac1f35eb37063e8ee7b
Reviewed-on: http://review.couchbase.org/39302
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>