avocado.core.parser: Add option --config.

Use command line --config to read a custom configuration from a file.
Note this option is for the avocado application, so it has
effect for all subcommands.

Example:

$ avocado --config /var/tmp/avocado.conf config
Config files read (in order):
/home/rmoura/Work/avocado/etc/avocado/avocado.conf
/home/rmoura/Work/avocado/etc/avocado/conf.d/gdb.conf
/var/tmp/avocado.conf
...

Also, update man-page about the new option --config.
Signed-off-by: NRudá Moura <rmoura@redhat.com>

avocado/core/parser.py

@@ -21,6 +21,7 @@ import sys
 import argparse
 
 from avocado.core import tree
+from avocado.core import settings
 from avocado.version import VERSION
 
 PROG = 'avocado'
@@ -45,6 +46,8 @@ class Parser(object):
         self.application.add_argument('--plugins', action='store',
                                       help='Load extra plugins from directory',
                                       dest='plugins_dir', default='')
+        self.application.add_argument('--config', metavar='CONFIG_FILE',
+                                      help='Use custom configuration from a file')
 
     def start(self):
         """
@@ -55,6 +58,10 @@ class Parser(object):
         """
         self.args, _ = self.application.parse_known_args()
 
+        # Load settings from file, if user provides one
+        if self.args.config is not None:
+            settings.settings.process_config_path(self.args.config)
+
         # Use parent parsing to avoid breaking the output of --help option
         self.application = argparse.ArgumentParser(prog=PROG,
                                                    description=DESCRIPTION,

man/avocado.rst

@@ -6,7 +6,7 @@
 SYNOPSIS
 ========
 
- avocado [-h] [-v] [--plugins PLUGINS_DIR]
+ avocado [-h] [-v] [--plugins PLUGINS_DIR] [--config CONFIG_FILE]
  {run,list,sysinfo,multiplex,plugins,datadir} ...
 
 DESCRIPTION
@@ -31,6 +31,7 @@ on them being loaded::
  -h, --help             show this help message and exit
  -v, --version          show program's version number and exit
  --plugins PLUGINS_DIR  Load extra plugins from directory
+ --config CONFIG_FILE   Use custom configuration from a file
 
 Real use of avocado depends on running avocado subcommands. This a typical list
 of avocado subcommands::