Add support for password protected key file to ssl_client2
Add support for password protected key file to ssl_client2

The example application programs/ssl/ssl_client2 allows the
configuration of a client CRT through the parameters
- crt_file, key_file
However, password protected key files are not supported.

This commit adds a new command line option
- key_pwd
which allow to specify a password for the key file specified
in the key_file parameter.

ARMmbed/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls