PythonVirtualenvOperator with provide_context=True does not have 'ti' keyword

**Apache Airflow version**: 1.10.13

**Environment**: Docker (Ubuntu 18.4 - Python 3.7)

- Cloud provider or hardware configuration: Azure
- OS (e.g. from /etc/os-release): Docker (Ubuntu 18.4 - Python 3.7)
- Kernel (e.g. uname -a): Docker (Ubuntu 18.4 - Python 3.7)
- Install tools: N/A
- Others: N/A

**What happened:**

When we enable provide_context=True for PythonVirtualenvOperator and try to use the xcom_push to pass a variable I get this error:

`File "/tmp/venv0upgqome/script.py", line 13, in push\n kwargs[\'ti\'].xcom_push(key=\'value from pusher 1\', value=value_1)\nKeyError: \'ti\'\n'`

**How to reproduce it:**

```
import airflow
from airflow import DAG
from airflow.operators.python_operator import PythonVirtualenvOperator

args = {
'owner': 'Airflow',
'start_date': airflow.utils.dates.days_ago(2),

}

dag = DAG('BAtatas', schedule_interval="@once", default_args=args)

def push(**kwargs):
"""Pushes an XCom without a specific target"""
value_1 = [1, 2, 3]
print("printing the kwargs!!!")
print(kwargs)
kwargs['ti'].xcom_push(key='value from pusher 1', value=value_1)

def push_by_returning(**kwargs):
value_2 = {'a': 'b'}
"""Pushes an XCom without a specific target, just by returning it"""
return value_2

def puller(**kwargs):
value_2 = {'a': 'b'}
value_1 = [1, 2, 3]
"""Pull all previously pushed XComs and check if the pushed values match the pulled values."""
ti = kwargs['ti']

# get value_1
pulled_value_1 = ti.xcom_pull(key=None, task_ids='push')
assert pulled_value_1 == value_1

# get value_2
pulled_value_2 = ti.xcom_pull(task_ids='push_by_returning')
assert pulled_value_2 == value_2

# get both value_1 and value_2
pulled_value_1, pulled_value_2 = ti.xcom_pull(
key=None, task_ids=['push', 'push_by_returning'])
assert (pulled_value_1, pulled_value_2) == (value_1, value_2)

push1 = PythonVirtualenvOperator(
task_id='push',
dag=dag,
python_callable=push,
requirements=[],
python_version='3.7',
use_dill=False,
provide_context=True,
system_site_packages=True,

)

push2 = PythonVirtualenvOperator(
task_id='push_by_returning',
dag=dag,
python_callable=push_by_returning,
requirements=[],
python_version='3.7',
use_dill=False,
provide_context=True,
system_site_packages=True,

)

pull = PythonVirtualenvOperator(
task_id='puller',
dag=dag,
python_callable=puller,
requirements=[],
python_version='3.7',
use_dill=False,
provide_context=True,
system_site_packages=True,

)

pull << [push1, push2]
```