Added 'environment_variables' capability

Added 'environment_variables' capability

- Environment variables are applied to sdb shell process environment.
- Each environment variable should be separated by new line character('\n').
  e.g.) A=B\nC=D
- The length of environment string should be less than 4096 byte.
- The total number of environment variables should be less than 100.

Change-Id: If5b52217b364ecf45eb88f0d8d3644cec40d3d30
Signed-off-by: shingil.kang <shingil.kang@samsung.com>