Add greentea-client dependency detection

[stegut01]

Description

Added detection of greentea-client dependency. Print warning if greentea-client is not in dependencies and abort test.

Fixes issue #84

Example

Example shows console output if greentea-client is not specified.

$ mbedgt -VS
mbedgt:
        *****************************************************************************************
        * We've noticed that NO 'greentea-client' module is specified in                        *
        * dependency/testDependency section of this module's 'module.json' file.                *
        *                                                                                       *
        * This version of Greentea requires 'greentea-client' module.                           *
        * Please downgrade to Greentea before v0.2.0:                                           *
        *                                                                                       *
        * $ pip install "mbed-greentea<0.2.0" --upgrade                                         *
        *                                                                                       *
        * or port your tests to new Async model: https://github.com/ARMmbed/greentea/pull/78    *
        *****************************************************************************************

mbedgt: completed in 0.00 sec

[PrzemekWirkus]

You need to more verbose:

gt_logger.gt_log("""
*****************************************************************************************
* We've noticed that 'greentea-client' module is specified in dependency/testDependency *
* section of this module's 'module.json' file.                                          *
*                                                                                       *
* This version of Greentea doesn't support 'greentea-client' module.                    *
* Please downgrade to Greentea before v0.2.0:                                           *
*                                                                                       *
* $ pip install mbed-greentea<0.2.0 --upgrade                                           *
*                                                                                       *
* or port your tests to new Async model: https://github.com/ARMmbed/greentea/pull/78    *
*****************************************************************************************
""")

[PrzemekWirkus]

On Linux this works:

$ sudo pip install "mbed-greentea<0.2.0" -U

See quotes around greentea module name and version.

[PrzemekWirkus]

Changed also on Windows:

$ pip install "mbed-greentea<0.2.0" --upgrade
...
$ mbedgt --version
0.1.20

$ mbedhtrun --version
0.1.24

+1