Add option to skip release discovery

`rlx_prv_discover` provider recursively scans all directories in
`lib_dirs` list looking for `*.app` and `*.rel` files.

This commit adds an option `disable_rel_discovery` to skip searching for
`*.rel` files when we know there aren't any. This speeds up release
generation with a lot of dependencies on the `libs_dir` list
significantly and reduces total release generation time in half.
Enable cover in CT tests
Merge pull request #98 from konradkaplita/enable_cover_in_ct

Enable cover in CT tests
Accelerate `rlx_prv_discover` provider

Introduce option `enable_shallow_app_discovery` which is disabled by
default.

When enabled searching for `*.app` files is done by using
`filelib:wildcard/1` and doesn't recursively traverse all directories
under `libs_dir` root dirs.

Instead its assumed that each directory on `libs_dir` list contains a
flat list of application dirs or is an app itself.
don't output release successfully created if verbose option is 0
fix path to start_clean in console_clean start script option
add config option for output_dir, currently only a cmd arg
mv nodetool and install_upgrade script to bin dir, so no need to incl erts
Merge pull request #96 from konradkaplita/disable_rel_discovery

Add option to skip release discovery
Merge pull request #110 from tsloughter/console_clean

fix path to start_clean in console_clean start script option
Merge pull request #111 from tsloughter/config_output_dir

add config option for output_dir, currently only a cmd arg
Merge pull request #109 from tsloughter/verbose_opt_fix

don't output release successfully created if verbose option is 0
properly handle config files and switching on/off dev_mode
Merge pull request #113 from tsloughter/dev_mode_sys_config

properly handle config files and switching on/off dev_mode
Merge pull request #112 from tsloughter/mvoe_nodetool_install_upgrade

mv nodetool and install_upgrade script to bin dir, so no need to incl erts
set ERL_LIBS when running console | console_clean | console_boot
Merge pull request #116 from soundrop/fix/console-path

export ERL_LIBS before running console|console_clean|console_boot
Run the node embedded mode when in foreground
Merge pull request #117 from soundrop/fix/foreground-embedded-mode

Run the node embedded mode when in foreground
Added R16B01/R16B02 otp release to travis config