Http Server: Add a file parser

When the http server reads a file from the disk, it is sometimes
required that the files will be parse and string substitution will be
perform.

For example, when reading the json files, it is required to place the
real host name/ip so the Swagger will be able to use those files. The
file parser will look for a parameter in the file (given that it has the
right extension) and will replace it.

Currently it only support {$Host} parameter, additional parameters can
be added when needed

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Merge branch 'master' of github.com:amnonh/mgmt